Every time I wish to add a part to a job I have a message appear which says (Error adding items). I can't add anything other than labour.

I have the steps you can perform to add items to a job in QuickBooks Online (QBO), @info908. 

The stored cache on the browser may be full, which causes unusual behavior in QBO. That's why you're getting an error each time you add a part to a job. 

To help resolve this, I'd first suggest logging into your QBO account using a private browser. It doesn't store data in the cache. Thus, it's a great way to isolate browser-related issues. If you're using Mozilla Firefox or Microsoft Edge, press CTRL + Shift + P on your keyboard. For Safari, hit Command + Shift + N. Press CTRL + Shift + N if you're using Google Chrome. 

Once signed in, go back to the Projects menu and select a job. If you're able to add items without an error, switch back to your main browser. Then, clear its cache to remove the junk files on the browser. This way, it will function efficiently.

However, if the error persists, please use another supported browser. 

Additionally, you can always run any reports from the Project Reports tab. These will help you track your expenses, profits, and time costs by project. Just choose a report and click View within its section. The screenshot below serves as your visual guide.

Please know that I'm always here to help if you need anything else. Enjoy the rest of your day, @info908.