'Sch C line 51-52 are calculated (not editable) and causing error. How to correct?'

Report the trade-in of the vehicle as a sale.  If you are using TurboTax online, from the Your income and expenses screen, scroll down to Other Business Situations and select Sale of Business Property.  Then enter your information. 

If you are using TurboTax CD/download, then from the Wages & Income tab, scroll down the screen to Business Items and select Sale of Business Property. 

If you obtained another vehicle for use in your business in connection with the trade-in of the old vehicle, then your basis in the old vehicle is what you paid for it less depreciation--if you took depreciation--or the depreciation portion of the standard mileage rate if you took standard mileage. The basis in the new vehicle is the cost of the new vehicle.  The amount you received for the old vehicle is not relevant, because there is no exchange, just a sale and a purchase. 

Because the vehicle was mixed use, you need to make the appropriate allocations when entering the information about the sale of the vehicle.  

@wileyrb