I was married on December 12th 2020 and have not yet received my legal marriage documents (as of Jan 2021). Is it still ok to file my taxes for the 2020 year as single?

[SweetieJean]

No. You must file as Marred. Be sure the Social Security Admin has your correct name now.

[DoninGA]

If you were legally married according to the laws of your state you can only file as Married on the 2020 tax return.  You would want to file as Married Filing Jointly even if one spouse has little or no income.