Insufficient information to know why. Best you contact IRS. My wife had a similar issue a few years back and it was a case of identity theft. Turned out to be a nuisance but no loss to us. Good luck.
If you used the IRS non-filer site to get a stimulus check, you will now get a “duplicate SSN” rejection. We are seeing people who did not notice or ignored the warning on that site about using it if they would be filing a tax return.
It could also be if you signed up on the IRS non-filer tool to give the IRS your Direct Deposit Info for the Stimulus Check. If you did then see
Or did you already file your personal return? Are you trying to just file business schedule C? That needed to go on your personal 1040 return. So you need to amend to add it.
Or what kind of business return do you need to file? You might need Turbo Tax Business. The small business program (TT Business) is for 1120 Corporation, 1120S Corp, 1065 Partnership or 1041 Estate/Trust returns and will not do your personal 1040 return. It is a separate program from the Desktop Home & Business program or the Online Self Employed version.
Turbo Tax Business is not available to do online or on a Mac. You can buy the Window's version here. And you can have both TT Business and TT Home & Business (or any personal version) installed on your computer at the same time.