The total sales price should be allocated among all the assets (land, rental property, improvements, etc.)

 If you have already allocated 100% of the sales price, then enter zero [entering the "0" will trigger the calculations] for these remaining assets and improvements.

Enter the date of sale (same as the rental property) for each asset/improvement.

Turbotax will automatically "write off" the remainder of the basis (cost minus depreciation). You don't have to manually do that.

You have to edit each asset that is listed for the rental property and enter the same date and the zero in the the sale section to have each asset to be used in the calculations.