Level 15

What's missing from your story is how you went back to Social Security, after receiving your green card, to apply for a new Social Security card (same number) that showed you are eligible to work.  Social Security has a "Numident" system (sounds like toothpaste) that keeps track of such things.

It's quite possible that you have been able to work in the U.S. for all these years without Social Security updating your employment eligibility because most employers didn't (and still don't) care about such niceties.  But thank the EIP program for bringing the problem to your attention.  As pointed out elsewhere, you're eligible for a credit on your 2020 income tax return, if you meet the other requirements.