I understand the significance of making an invoice in QuickBooks Online (QBO), hillswindowclean.
There are times that browser is full of frequently accessed page resources, which causes some errors or unusual responses. With this, we can perform some troubleshooting steps to fix this.
Let's start accessing your account using a private browser. You can use either of these shortcut keys:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Microsoft Edge: Ctrl + Shift + P
Safari: Command + Shift + N
If you can now create an invoice after this, then the problem is most likely caused by your browser. You'll then can clear your regular browser cache. If the issue persists, you may need to use another supported browser.