Quick answer
Folly is genuinely fun to the man who lacks judgment, and that enjoyment is the problem. He is not enduring his foolishness; he is enjoying it.
Read both lines
The NKJV footnotes "discernment" as literally heart, the book's idiom for the whole thinking, deciding center, so the first line describes a man missing his interior compass and delighting in the fact. The verse refuses the comfortable assumption that foolish living is secretly miserable. Often it is a good time, and that is exactly what makes it durable, since nobody abandons what they are enjoying. The second line offers no competing thrill, only a direction: the man of understanding "walks uprightly," straight-ahead motion instead of entertainment. The chapter will return to this heart language at verse 32, where hearing correction is literally acquiring a heart, which is precisely what this man is missing.
Related: Proverbs 15:32 · Proverbs 10:23 · Proverbs 14:12
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.