Let's work together to ensure you'll be able to send invoices.
Can you please confirm what specific error message you're seeing when trying to send an invoice? Any additional information will help me check if other users are having the same issue.
You can try sending an invoice using a private browser to identify if the error isn't a browser-related issue. You can use the following shortcut keys:
Safari: Command + Shift + N
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
If you can send the invoice successfully using a private browser, you can go over to your regular browser and start clearing the cache. This eliminates excessive files which result in unusual responses in QuickBooks Online. Otherwise, you can use other supported browsers.