As you are on a cash basis, income received in 2021 is only reportable on your 2021 tax return (next year) even if the payment was for work done in 2020.
**Mark the post that answers your question by clicking on "Mark as Best Answer"
The IRS taxes you in the year income is received, so as Tax Expert @MinhT1 the money should be taxed in 2021. Double-check with HR to make sure they are not going to issue a 2020 W-2 or 1099-NEC.
They should not, but you said there were some hiccups so it's worth checking.