Assuming you are using cash (not accrual) basis for the business. Should be taxable in the year when you could access to the money.
See Publication 17 (2021), Your Federal Income Tax | Internal Revenue Service (irs.gov)
If the money was made available to you in 2021 but you did not pick up a check until 2022, or did not cash it until 2022, or the delay in accessing the funds made available to you in 2021 is under your control, its taxable in 2021.
