What does the error message say? Please clarify.
Within the establishment of the vehicle within the self-employment activity, did you select I own this vehicle? Or something else?
The hyperlink Why can't I take the standard deduction states:
You can't claim the standard mileage rate if you don't own the vehicle you used for work. However, you can still claim your actual expenses for things like gas, oil, and repairs. We'll walk you through all the vehicle expenses you can claim.