Ok ... you have 2 issues to address ... first the self employment on Sch C ... did you enter the 1099-NEC income anywhere on the return? Did you enter it on the Sch C or Sch 1 line 8 ?
And second the stimulus credit ... you can have either the stimulus credit OR the stimulus advance but not both. So if you have the stimulus amount on the form 1040 line 30 but the IRS thinks they sent the advance already then you need to check your records for the payment ... then if you really did not get it have a trace put on the missing money.
1) check you bank records from March - May, 2021 to see if you overlooked the fact that you received the 3rd stimulus already. if you find it, there is nothing else for you to do.
2) if there is no record of receipt, then you'll have to complete the trace document and submit
https://www.irs.gov/pub/irs-pdf/f3911.pdf
