If you had a lower-cost vehicle when you first set it up for Business, and used several years (or had many business miles) your accumulated depreciation might be more than the original cost of the vehicle, so TurboTax gives you an error.
If you think that is the case, report the Original Cost you entered for your Vehicle as both Depreciation and AMT Depreciation amounts. In FORMS mode, you can type the amount right on the Vehicle Expense Worksheet.
Depreciation is calculated differently, depending on whether you claimed Standard Mileage or Actual Expenses.
If you claimed Standard Mileage expense for your Business Vehicle each year, part of that deduction is the 'depreciation equivalent' (22 cents of the 55 cents per mile deduction, for example).
You would multiply the number of Business Miles each year times the Depreciation Equivalent for that year. Again, the depreciation can't exceed the Cost of the Vehicle when you originally started using it for business.
If you claimed Actual Expenses for your business vehicle each year, the Form 4562 Depreciation Schedule on last year's return will show Accumulated Depreciation.
[Edited 03/20/2020 | 7:49 PM]
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