Enter "$1" as the rental income.
This will not have any effect on your tax calculation and will not generate an error in either TurboTax or with the IRS. From an economic perspective, it is accurate - your 2020 rental income was virtually "0".
There are many places in TurboTax where you must make a non-zero entry, primarily to let the program know that you answered the question and that the answer makes some sense.
In this case, entering "$1" tells the program that you didn't forget to answer the question and that you had at least some rental income in 2020 to justify your expenses and deductions. You can also enter a "$1" miscellaneous expense ("Rent offset") to correct the overstatement of rental income.
(As @ColeenD3 stated, It becomes rental property "the moment it is placed in service and ready and available to rent." Once "placed in service", you can claim all related rental expenses.
