Turbotax is asking "What was the fair market value of (a property)? Is this when I purchased it or when I rented it?

You want to enter the fair market value of the property when you placed it in service, i.e. when you started renting it.  This is when depreciation starts, so if there were any improvements on the property, the basis will be higher than the purchase price.

For more information, see Tax Deductions for Rental Property Depreciation - TurboTax.