I want to provide you with accurate assistance, and to achieve this, may I ask what's the error message you've received when sending your invoice? Any additional details would help us resolve this matter accordingly.
In the meantime, we can open your QuickBooks Online (QBO) account using a private browser. It could help us detect any existing issues with your regular browser.
Here are the keyboard shortcut keys for opening a private session:
Safari: Command + Option + P
Google Chrome: Ctrl + Shift + N
Firefox or Internet Explorer: Ctrl + Shift + P
If you don't encounter any errors when sending an invoice using a private session, it indicates an issue with your regular browser. Let's fix this by clearing your browser's cache and cookies.