This is one of the most MISiNTERPRETED verses in the Bible. And it has also been corrupted in the Critical Text and the modern versions.
CRITICAL TEXT (mutilated verse)
καὶ οὐδεὶς ἀναβέβηκεν εἰς τὸν οὐρανὸν εἰ μὴ ὁ ἐκ τοῦ οὐρανοῦ καταβάς, ὁ υἱὸς τοῦ ἀνθρώπου. [ὁ ὤν ἐν τῷ οὐρανῷ omitted]
RECEIVED TEXT
καὶ οὐδεὶς ἀναβέβηκεν εἰς τὸν οὐρανὸν εἰ μὴ ὁ ἐκ τοῦ οὐρανοῦ καταβάς ὁ υἱὸς τοῦ ἀνθρώπου ὁ ὤν ἐν τῷ οὐρανῷ.
Literal rendering: And no man has gone up into Heaven if not the [One] out of Heaven having come down, [even] the Son of Man who is in Heaven.
KING JAMES BIBLE
And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.
Firstly, what is the context of this verse? Jesus was teaching Nicodemus the absolute necessity of the New Birth, and speaking about heavenly things. Christ went on to assure him that only He who had come down from Heaven could testify to these heavenly things which He not only knew but had also seen.
It is in this context that Christ speaks of His UNIQUE ASCENSION into Heaven. He is prophesying about Himself, that after His death, burial, and resurrection, He would ascend to Heaven as the resurrected one who had come down from Heaven.
But He did not stop there. He added something at the end to show that He is God. While the Son of Man is on earth, He is also in Heaven in the Spirit. So Christ was always in two realms at the same time.
Therefore this is not a denial of the ascension of Enoch and Elijah (which almost everyone concludes) but it is a revelation of the uniqueness of the Son of God who is also the Son of Man.