If it is being converted to 100% rental, the "placed in service" date is when it is advertised, ready, and available to be rented and you need to enter ZERO personal days.

Expenses after that date go on Schedule E as deductions, expenses before that date are usually capitalized and become part of the Basis for depreciation.

It is was "placed in service" in 2020, you enter ZERO personal days.  With zero personal days, the program should not give you any problems with zero rental days.  Manually prorate annual expenses such as mortgage interest, real estate taxes and insurance based on the "placed in service" date..