They need your Social Security number in order to issue your W-2. They should have gotten it when you started working there. If you did fill out a W-4, you should have put your Social Security number on the W-4, so they shouldn't need a W-9.
Since asking an employee for a W-9 is unusual, you should probably talk to them to clarify why they are asking for a W-9. You want to make sure that they are going to report your income on a W-2 and not a 1099-NEC or 1099-MISC.
