What does God made Jesus to “be sin who knew no sin” mean? (2 Corinthians 5:21)

The simple answer is, just what it says. This is clearly a somewhat figurative message since ‘sin’ is concept like ‘wrongdoing’. And a human being cannot literally become sin (5:21a), any more than someone can “become the righteousness of God” (5:21b). Nor can a human literally become truth or justice or any other concept or quality. But the text is what it is, and Paul says that God made Jesus, who was “tempted like us in all points like us but without sin” (Hebrews 4:15, cf. 2:18), actually “become sin”. Which is not meant to be an easy idea.Some readers have not unreasonably wondered;

Could it be read “made Jesus to be a sin offering”?

To which the answer is; no. The Greek clearly and unambiguously says  ἁμαρτίαν ἐποίησεν – “made sin”. Sin offering in Greek is a different phrase = to (the thing) peri (for) hamartion (sins). If Paul had wanted to say that he could have done. And indeed the New Testament elsewhere does say that Christ made an offering or sacrifice for sins – Hebrews 10:12, 1 Peter 3:18, 1 John 3:5 among many examples. But 2 Corinthians 5:21 is saying something different:

God made Christ X, that we might become Y.

We cannot literally become the righteousness of God. Christ did not literally become sin. But God in putting Christ in the place of sin was encouraging us to change and be reconciled to our Creator, making an effort to try to reflect God’s image by righteousness.

As always with Paul in his epistles the subtext is motivation to imitate Christ in thinking. speech and behaviour.