To ensure that I'm on the same page, may I know if you're getting any error messages when adding a product item in QuickBooks Online (QBO)? Any additional information will help me provide the best resolution.
In the meantime, try logging into your QBO account using a private browser (incognito). There are times that the browser is full of frequently accessed page resources, causing some unusual responses.
Here's how:
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Once signed in, try to add a product again to double-check.
If it works, go back to your default browser and perform a clear cache. Every so often the cache becomes overwhelmed with older data which can potentially cause viewing and performance issues. Clearing it will refresh the system, and you'll be able to work with a clean slate. However, if the issue persists, try using other supported browsers.