[Heb 7:11 NLT] (11) So if the priesthood of Levi, on which the law was based, could have achieved the perfection God intended, why did God need to establish a different priesthood, with a priest in the order of Melchizedek instead of the order of Levi and Aaron?
The NLT is very paraphrastic. Here is my offering.
Hebrews 7:11-12 If indeed perfection was to be had in the Levitical priesthood (inasmuch as under the same the people were placed under the law), what need then was there that a priest arise after a different order, that of Melchizedek, and not rather one said to be after the order of Aaron? For with a change in priesthood, there is of necessity a change in law also.
The thrust of this argument is that in the Messianic psalm, there was to arise a priest who is said to be after the order of Melchizedek (Psalm 110:4), that is, not an Aaronic priest—his priesthood predated and was superior to the Aaronic priesthood by virtue of the Aaronic recieving blessing, via Abraham the progenitor of Aaron, from his (Genesis 14:18-20). The fact that God swore to make His Messiah a priest after the order of Melchizedek (argues the author of Hebrews), is sufficient cause for one to conclude that the Melchizedekian priesthood is superior in some sense to the Aaronic, by virtue of its being chosen over the latter for the Messiah to assume. Indeed, the Aaronic, being limited to a tribe, was a punishment for the disobedience of Israel when they prostituted themselves to the golden calf, and was not God's original intent; whereas Melchizedek, significantly, was a priest without respect to tribe or progenitors, inasmuch as in the text he appears apparently out of nowhere (something the author of Hebrews takes to typologically foreshadow Christ's perennial priesthood—Hebrews 7:3).
As far as the point about law as relates to the priesthood is concerned, the author draws attention to the fact that where there is no longer an Aaronic priest, but a priest after a different order, "there is of necessity a change in the law also," because the law was built around the Aaronic priesthood, not the "bread and wine" offering of Melchizedek, a clear pointing forward to the Eucharistic offering—one that will be made by Gentiles, called to partake in the priesthood of Christ, who is thus the highpriest (Luke 22:19; 1 Corinthians 10:16), and thus not by priests according to the order of Aaron (1 Corinthians 10:16; 11:29; Malachi 1:11-12)—meaning that the new Law necessarily follows from the different priesthood which is not Aaronic; and that it is preferred over the Aaronic in view of the Messiah, proves that the new Law is superior, and thus by extension that the former could not offer perfection (inasmuch as you cannot exceed perfection).