E-filing does not start until Feb 12th, so you have time to get it worked out. It is possible (and common) that your employer paid you some amounts in 2020 even though you quit in December of 2019. The only way to know for sure is to contact them. But if they issued you a 2020 W-2, the IRS will be expecting to see it on your tax return. Check with your former employer, you may need them to issue a corrected copy of the W-2.