Dep allowed/ allowable is too large. Depreciation cannot exceed business basis.

Purchased a business vehicle in 2017 and have claimed mileage deduction since putting the vehicle into service. Truck was used 100% for business (landscaping). At the end of 2020 we purchased a new business vehicle and sold the previous. The original cost of the truck was $12500. We deducted well over that in mileage deductions (around $18k from the included depreciation rate in the mileage rate.). We sold the truck for $3600. No matter what numbers I put down, when I go to file it claims the depreciation is too large. The only way it goes through is if I put 0, but I'm unsure if that will cause issues/is incorrect.