I got an error when trying to submit my taxes. It claimed my social/2019 taxes were already used to file, but I haven’t filed this year yet

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

https://ttlc.intuit.com/community/tax-topics/help/stimulus-how-to-file-your-return-after-getting-reject-r0000-902-01/01/1602609

If you're saying that your social security number has already been used to file (make sure you typed it in correctly) then you may well have been the victim of identity fraud. I believe you need to address it with an agent of the IRS directly to find out what steps you need to take from there.