Hi info2398, you can’t fully remove it in QuickBooks Self-Employed. The option to make it inactive only appears for a non-primary vehicle, so if it’s missing, that vehicle is likely set as primary.
In that case, add another vehicle and set it as the primary one first. Then go to Miles > Add Trip drop-down > Manage vehicles, select the car you want to inactivate, click Vehicle no longer in use, and then Save.

Please let us know if you have any other concerns.