Let's resolve the error you're having so you can add or edit a new product item, @sarahrayside.
There are times that the browser you're using has information stored that's stopping or causing the error when adding or editing an item.
I recommend using a private browser or an incognito window when trying to add it. Then, you may clear the cache in the one that you're currently using if it works. Here’s how:
If it works, here's how to clear cache and cookies to fix issues when using QuickBooks Online. Once you're done clearing your browser's cache, restart your browser for the changes to take effect. If not, try using other browsers.
Check these articles for more information:
I'm always here if you need help or assistance with adding your products and services. Just leave a reply below. Take care!