Trying to finalize filing and this alert keeps showing up, line 18 of form 1040 shows me nothing helpful
This is referring to the SSN of whoever you are paying alimony to. Specifically, the SSN is on 18b.
You might not be able to see the worksheet until you pay, if you are using the Online system.
Line 18 indicated that you entered that you paid alimony but failed to enter the SSN of the ex spouse that it was paid to.