Was there $600 on 1040 line 30 for the Recovery Rebate Credit? That was for the 2nd Stimulus check. The IRS has been adjusting returns that claimed it but the IRS already sent it. Did you get it after you filed?
Sorry you tried to claim it again on your tax return when you already got it. It reduced the tax due on your return. Your tax return should have been a 599 tax due instead of $1 refund. Sorry you owe the 600 back. You don't get it twice.
Amending wont help. If you take off the 600 credit on line 30 you are left with owing 599. Don't know why you owe for 2020. Your withholding must have gone down. You have to compare 2019 and 2020 to see what changed.
