Welcome to the community, Tom! :grinning_face_with_big_eyes: Unfortunately Airtable’s native forms don’t allow for the kind of logic that’s required for your scenario, where a selection in one list drives the options available in another list. What you want can be achieved, though, with the help of third-party form software that ties into Airtable, specifically the On2Air: Forms / Jotform combo.
Another option is to combine the category and job into a single line, which could be done with a formula in the primary field of the table used for selection.
It’ll mean a bit more scrolling when the user picks from the list, but it could be done directly within Airtable’s own forms.
Welcome to the community, @Robin_0829! :grinning_face_with_big_eyes: It’s easiest for us to help if you could share what you’re already doing, and we can offer specific guidance from there. If you’d like to hire a consultant to help build the system for you, please post to the #developers:work-offered category.