When you're setting up, you'll come across a section where you have to enter your company information.
Towards the bottom of the page, you'll see that you must assign a category. Within the drop down there are over 4000 different categories you can select from.
However, if you can't find a category that is specific to your business, then we advise that you choose one that is more general.
For example, there isn't an option for a "water heater installer". But they do have "plumber" who covers this service.
So, go more broad until you find something that is suitable.
The category is used to help drive the recommendations the platform makes around your keyword strategy and local SEO actions. So please take your time to find a category that suits your whole business.