Your misunderstanding what the writer of Hebrews is explaing.. Part of the law referred to is that which is under the lecitical priesthood and that included sacrifices.. jesus was not from the tribe of Levi and He Himself was the sacrifice. The law here has changed and referring back to the order of melchizedek.. Any how here is a brief explanation :
For the priesthood being changed - According to the prediction in Psalm 110. that it would be. When that occurs, the consequence specified will also follow.
There is made of necessity a change also of the law - The Law so far as it grew out of that, or was dependent on it. The connection requires us to understand it only of the Law "so far as it was connected with the Levitical priesthood."
This could not apply to the ten commandments - for they were given before the institution of the priesthood; nor could it apply to any other part of the moral law, for that was not dependent on the appointment of the Levitical priests. But the meaning is, that since a large number of laws - constituting a code of considerable extent and importance - was given for the regulation of the priesthood, and in reference to the rites of religion, which they were to observe or superintend, it followed that when their office was superseded by "one of a wholly different order," the Law which had regulated them vanished also, or ceased to be binding.
This was a very important point in the introduction of Christianity, and hence, it is that it is so often insisted on in the writings of Paul. The argument to show that there had been a change or transfer of the priestly office, he proceeds to establish in the sequel.(Barnes Hebrews 7:12).
This is why Jesus can say in Matthew:
For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. Matt 5:18.
The following is a part of a document that I created some time ago. I will probably post the entire document in another thread soon; but chapter 3 applies to the discussion at hand and I think that it may help you to understand where I am coming from.
3
1 It is clear that our Lord sprang out of the tribe of Judah, of which tribe Moses spake nothing concerning priesthood; and therefore in order to be High Priest, as He is High Priest, Jesus must be High Priest according to the order of Melchizedek.
2 And the priesthood being changed from Levitical to Melchizedekan, there is of necessity a change also of the law.
3 Therefore has even one jot or tittle passed from the law?
4 Every jot and tittle of the Old Testament remains inspired and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God might be perfect, thoroughly furnished unto all good works.
5 Every sacrifice points to the ultimate sacrifice of Christ on the Cross.
6 Every moral tenet speaks to me on how to more specifically love the Lord my God with all my heart, soul, mind, and strength, and my neighbor as myself.
7 The change in law therefore refers to the transition from looking to obey a set of do’s and don’ts, to walking according to the Spirit rather than the flesh.
8 For what the law could not do, in that it was weak through the flesh, God sending His own Son in the likeness of sinful flesh, and for sin, relegated sin to the flesh.
9 Therefore the law is now this: anything I do according to the flesh is sin.
10 If I give in to fleshly desire then I walk according to the flesh.
11 If I do anything in my own ability or strength, I also walk according to the flesh.
12 And if I do anything contrary to the love of God, I am also walking according to the flesh.
13 However the law of the Old Testament properly lays out, in its set of do’s and don’ts, what it means to walk according to the flesh if I were to disobey any of its tenets.
14 If I disobey a law in the Old Testament that is not fulfilled by the New (in that Christ is sacrificed for us), I am walking according to the flesh.
15 And therefore the Old Testament law is still valid as an old way of defining what it means to walk according to the flesh and to walk according to the Spirit.
16 By the Old Testament law is still the knowledge of sin.
17 Sin is still the transgression of the Old Testament law.
18 And yet in the New Testament we also find moral tenets given as a set of do’s and don’ts that also tell us what it means to walk according to the flesh and according to the Spirit, defining sin for us.
19 Because of certain things the wrath of God falls on the children of disobedience!
20 We are to mortify such things as fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry.
21 Also no whoremonger nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God.
22 Therefore the New Testament is not nullified by the Old, and neither is the Old Testament nullified by the New: but the moral tenets of both together produce a codified law.
23 And this is what it means when “the law” is mentioned in scripture: the combination of moral tenets found in the Old Testament and the New.
24 The law says to those who are under it that they are not righteous; sinners in need of a Saviour.
25 But sin shall not have dominion over you and you are righteous if you are in Christ: for you are not under the law but under grace.
26 In that you subject yourself willingly to the law of God, being spiritually-minded, you are under the law to Christ: and this is the exception to the rule, because you are under the law not by obligation but by free will; and therefore you are righteous in Christ because you are at the same time not under the law.
27 Is Christ therefore become of no effect to you because you are not under the law but under grace? Sin shall not have dominion over you; are you not therefore justified according to the law in due process of time?
28 As it is written, “For not the hearers of the law are just before God, but the doers of the law shall be justified.”
29 And yet in another place it is also written, “Christ is become of no effect unto you, whosoever of you are justified by the law; you are fallen from grace!”
30 Therefore I am justified before God by faith in Christ, which produces the obedience whereby I am justified before man according to the law.
31 God knows that I am the chief of sinners: And that I am made righteous through His blood, and that this changes me from the inside out so that I become blameless in the sight of God and man.
32 Therefore in the sight of God I am not justified by the law but by faith in Jesus Christ: for He sees what is invisible and what can become visible because the effect is not very far from its cause.
33 Now I am face to face with the wet paint principle once again, if I begin to think that I am justified by the law.
34 Wherefore, since grace makes me blameless according to the law, I find that I am justified according to the law because of grace.
35 I must therefore consider that the root is grace; and the fruit obedience to the word of God.
36 And if I were to make the root of grace obedience and the fruit of obedience the grace of God in my life, I would have the cart before the horse and would indeed be justified by the law.
37 But because the root is grace, truly by grace am I justified before God and am justified by the law only in the sight of man.
38 And God alone sees the whole of the reality: for man looks on the outward appearance, but God looks on the heart.