Cash basis accounting is assumed for most small tax payers, income and expense are reported at the time of receipt and payment.
Accrual basis account has to be specially elected if your business grew to a certai size ($xx million revenue?) . In Accrual accouting, you report income when you earned it (e.g. goods and services delivered).
This is just a long winded way to say, you have to report all prepaid rent as 2020 income.
