Nothing special here. Enter your rental income for 2020 as $0 and that takes care of the income section entirely. Then enter your rental expenses, and press on.
Take note that it is NOT common for rental property to actually show a taxable profit anyway - especially if there's a mortgage on the property. Since your rental expenses can only be deducted from rental income, and since you have no rental income to deduct those rental expenses from, your "unallowed" rental expenses will be carried forward to the next tax year and included in that carry forward amount will be the total of all unallowed losses (if any) from all previous tax years.
