Hi there, marktree. We can take a few steps together to troubleshoot and regain your ability to add products or services to your invoices in QuickBooks Online (QBO).
 
Sometimes, browsers can accumulate frequently accessed pages, which could lead to unusual behaviors when adding products or services to invoices. We can click the Refresh icon, press F5 (on Windows), or Command-R (on Mac) to reload the page. 
 
Once refreshed, check if you can add products or services as expected. If the issue continues, sign out of your QBO account completely. Wait a few moments, then reconnect by logging back in.
 
Alternatively, we can open an incognito window in your browser to access the product with a clean slate. You can also switch to other supported browsers for better compatibility.
 
On another note, if you meant you're unable to add a new product or service directly from the invoice page, you can create the item by going to the Gear icon, then Products and services.

 
Don't hesitate to drop a comment below if you have further questions about managing invoices in QuickBooks. Know that I'm always ready to assist you.