Actually it is a commandment in the New Covenant Scripture.
If we pick apart the word Paul used, it can have the sense of "don't be deficient" in prayer.
NKJ 1 Corinthians 14:37 If anyone thinks himself to be a prophet or spiritual, let him acknowledge that
the things which I write to you are the commandments of the Lord.
- At times Paul made it clear that he was giving our Lord's commands
NKJ 1 Corinthians 7:25 Now concerning virgins:
I have no commandment from the Lord; yet I give judgment as one whom the Lord in His mercy has made trustworthy.
- At other times Paul made it clear that he had no command from the Lord so was functioning in his Apostolic role and judgment.
NKJ 1 Thessalonians 4:2 for you know what
commandments we gave you through the Lord Jesus.
- In 1Thess Paul says he gave commands through the Lord Jesus
Some of these words have some range, but that range does include "commandments"