How do I enter a payer's truncated social security number on a 1099-NEC?

Just enter it as Other self employment income or as Cash or General income.  You don't need to get a 1099NEC or 1099K.  Even if you did you can enter all your income as Cash.  Only the total goes to schedule C.

How to enter income from Self Employment

https://ttlc.intuit.com/community/self-employed/help/how-do-i-report-income-from-self-employment/00/26653

Thank you! Just to clarify, I did receive the 1099-NEC from a payer. If I enter it as general income and not under a 1099, would that raise a red flag with the IRS? And would it be best to lump it in with cash/checks or to enter it on a new line, perhaps even noting it was from a truncated 1099?

As already stated the IRS doesn't care ... all they see is the total income on the Sch C ... all that 1099 entry business in the TT program is just busy work and none of it goes to the IRS since the issuer already sent that info directly to the IRS.