I am sorry, that algorithm created for not permit two users at one time change some data.
So if you open some vehicle category - and not cloase it and leave opened - and close browser, Category will mark as checkout - and only you will can edit it.
Please try close ever category what you open for edit.
Now you or need login under users what edited that category or you need go to database - open vehicle category table and clean field Checkout - for that category
The administrator has disabled public write access.