My husband died in 2020. I am filing as single in 2021. My TT Form SSA-1099 is correct but shows a red ! with yellow highlighting the Spouse column. How can I fix this?

Most of the time, you need to make a new account when changing from married to single, because there are leftover forms and worksheets for your spouse that are hard to get rid of.  You can try going to the social security summary screen where it should list both your SSA-1099 and your spouse's SSA-1099, with a note next to your spouse's that says something like "needs review."  You should be able to delete the form from this summary screen with the trash can icon.  That may remove the red flags from the summary page.  If not, you may need to make a new account and start over as single.