According to David in Psalm 32:2 you are blessed when the Lord doesn’t impute iniquity to you...
When he doesn’t count it against you. When he doesn’t charge it to your account, as it were. Those are some synonyms of "imputeth not iniquity."
This imputing is what God did to Abraham in Genesis 15:6 when Abraham believed God and God in turn counted it unto him as righteousness. So the object that is imputed in Genesis 15 is quite different there than it is in Psalm 32.
In Psalm 32 the Lord does not impute iniquity. In Genesis 15:6 the Lord does impute righteousness.
And there’s some reason to think that both approaches are the interrelated. In other words, to negatively not impute iniquity is to positively impute righteousness. That’s what Paul says in Romans 4. He says that David in Psalm 32 “describeth the blessedness of the man, unto whom God imputeth righteousness without works…”
But David doesn’t say that in Psalm 32. He speaks of not imputing iniquity.
And yet, under the inspiration of the Holy Spirit, Paul says that when David here speaks of not imputing iniquity, it’s practically the same thing as speaking of imputing righteousness.