When I go to transmit my taxes it says my social security number has already been used to file, but I didn’t file taxes yet this year. Is it because I applied for the impact check?

Yes.  That filed a return for you.  See reject error FAQ due to Stimulus

https://ttlc.intuit.com/community/tax-topics/help/turbotax-online-rejected-fw2-001-02-and-or-f1040-034-05/01/1652494

Sorry there's a new article, see other answers.

That is the reason.  There was a warning on that screen that told you not to use it if you would need to file a tax return.

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

Sorry there's a new article 

If your return rejected because you filled out the IRS non-filer page see

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

I have a similar problem-trying to file electronically-  says social security number already used.  I did receive stimulus money, but had NOT applied for it.  What now?