Why was the law unable to do what it needed to do in Romans 8:3-4?
The law was righteous and perfect from God's standpoint, yet it was weak through the flesh—from man's fleshly side, man's point of view. Because of that weakness, the law could not do what was needed; God sent His Son in the likeness of sinful flesh to condemn old Adam in the flesh.
Les explains that the law itself was righteous and perfect from God's perspective, and always will be, but it was weak from the fleshly side, from man's point of view. He uses the term old Adam for the singular word sin throughout Romans, because old Adam is the fountainhead of all our problems, the Adamic sinful nature. The fruit of old Adam are the things we do, which are sins, plural. God sent His own Son in the likeness of sinful flesh, and for sin, to take the place of old Adam so that we could be crucified with Him, and condemned old Adam in the flesh. So the law could not do what was needed because the weakness was on the fleshly side; God accomplished it through His Son. Verse 4 then continues with the righteousness of the law, reminding us that the law was righteous.
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, condemned sin in the flesh:
That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
More Questions
Why could the law not do what God required?
What the law could not do, it could not do because it was weak from the fleshly side, from man's point of view.