If the property was advertised and available for rent, but just not rented during 2020, then it is considered to be a rental property that has been placed in service. If that is the case, then you can claim expenses for maintaining and repairing the property, mortgage interest, property taxes, and depreciation of the property and improvements.
If you purchased the property and have not placed it in service as a rental property, then expenses you incurred to maintain the property are not deductible as a rental expense. Major improvement costs for getting the property ready to rent can be added to the basis of the property and will be deducted through depreciation after the property is placed in service. Expenses such as mortgage interest (if this can be considered a second home for you) and property taxes may be deductible on Schedule A Itemized Deductions without the property being placed in service as a rental.
You will include Schedule E Rental Income and Expenses as part of your tax return in the year the property is placed in service (advertised and available for rent).