We could not add lines in the invoice, please check and update.

Welcome to the Community, @info1285.

This could happen if the browser is already full of frequently accessed page resources (cache and cookies), causing some unusual responses in QuickBooks Online (QBO). To isolate this, try signing into your QBO using a private browser (incognito).

Here's how:

• Press Ctrl + Shift + N (Google Chrome).
• Ctrl + Shift + P (Firefox).
• Control + Option + P (Safari).

Once logged in, try to add line items in the invoice again to double-check. Here's how:

1. Go to the + New option at the top left to create an invoice.
2. Click on the last box/line under Product/Service to add a row to add more lines. 

If it's successful, go back to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.

Please let me know how it goes or if you have any other issues or concerns in the comment section below. I'm always here to help. Have an amazing rest of the day!