If you have not filed yet, go to the personal information section and change from single to married filing separately. Give the name and date of birth of your spouse and leave the SSN blank. On the next page, check the box for "non-resident alien." On the page after that, when the program asks if your spouse had income or will file a tax return, this is only asking if your spouse had US-source income or will file a US tax return. If your spouse had overseas income and will file a tax return in their home country, but did not have US income and won't file a US tax return, you still answer "no."
Make any other changes needed. You can't e-file. Print your return and mail it to the IRS. Use a mail service with tracking and proof of delivery. If you owe tax, don't include a check. Instead, make an online payment at www.irs.gov/payments, be sure to select "2021 form 1040" as the reason.
If you already filed and need to file an amended return, wait for your first return to be processed and any refund to be paid. Then go into Turbotax and indicate you need to file an amended return. Follow the steps above.
In past years, you had to use a fake SSN to get past the error check. If this is still true, try 999-88-9999. After you print your return you have to use some white-out to cover the fake SSN and write "NRA" over it. (Turbotax asking if your spouse is NRA is new, so maybe the program now includes "NRA" automatically.)
