This guide explains the method to fix the cant modify vehicle bug in the Auto Shop. You just need to select an unpopular job from the online menu and leave it as soon as you enter it. The method to resolve this issue is explained in detail here.
Normally when you enter your vehicle, it would just ask you to press the d-pad and you can easily modify the vehicle. But in this case, you are unable to have access to the update menu. You just simply need to move to the online menu and then play jobs, Rockstar created. And you are needed to pick any unpopular job.
Now you just need to load into this job and then get out of it as soon as possible. You will be transferred to your auto shop on its roof or on the gate. After that, you just need to enter back into your auto shop.
Now you have to move to the vehicle in your garage which you want to update and it is highly likely that now you will have access to all the options to modify the vehicle’s bug. The basic solution is to just find an unpopular job and leave it just after entering into it.