Sold a car where business use varied. Entering 4797 manually., Calculated loss is correct. When I choose 1245 property, if shows error on line 21.

The best way to enter the sale of your business use vehicle is using the steps below. If you used the standard mileage rate (SMR) the chart is attached below to calculate the depreciation component for all business miles.  If you used actual expenses gather your information for all of the depreciation used as an expense on your returns from the time you began using it until the day you sold it.

1. Total all of the business miles for the life of the vehicle
2. Total all miles on the vehicle for the life of the vehicle
3. Business miles divided by total miles = Business Use Percentage for the life of the vehicle
4. Take the original cost of the vehicle x the business use percentage = Business Cost
5. Take the sales or trade-in price x the business use percentage = Business Sales Price
6. Calculate all of the business miles by year with the rate in the SMR for depreciation (chart attached) to arrive at the total depreciation considered used by the IRS; OR total all depreciation expense for all years, including the current year.
7. When you go through the vehicle asset information be sure to select 'Sold, disposed of, etc....' then do not indicate it was sold.  You must say 'Yes' it was converted to personal use.  This will eliminate any sales information in the vehicle it self.

Next enter your Sale of Business Property (do not enter any sales information in the vehicle asset) - This will automatically go to Form 4797.

1. Income at the top
2. Scroll down to Other Business Situations
3. Select Sale of Business Property
4. Select Sales of business or rental property that you haven't already reported.
5. Answer 'Yes' to Do all of the following apply...?
6. Enter your sales information, do not make an entry for depreciation (no zeros)
   1.  Description of the Property (Vehicle made/model/year)
   2. Sales Price/Sales Expenses 
   3. Date acquired and date sold
   4. Cost
   5. Depreciation