Stimulus checks are not taxed. If you did not receive the payments, you can apply for the Recovery Rebate Credit. This is added to your tax refund. If you owe any taxes, it must be applied to the taxes first.
When you started your tax return Turbo Tax assumed you did not get any stimulus checks so it gives you the full Stimulus as a refund on your tax return or reduced a tax due. Then at the end you enter you already got it so it takes it out of your refund. Because you already got it. You don't get it twice. Or if you only got the first 1200 you still get the second 600. Check 1040 line 30 for any amounts you didn't get during the year.
When you entered 0 for any stimulus checks you got or didn't get then it gives you the full amount you qualify for on your return which reduces the tax due. But if you already got some stimulus checks you don't get it again. Looks like based on your 2020 income you only qualify for a total of $558? If you already got more you don't have to pay it back.