Let's perform some troubleshooting steps so you can create a new invoice successfully.
Before we start, have you receive and error message upon creating an invoice? Any additional information would be greatly appreciated.
There are times that the browser is full of frequently-accessed page resources, thus causing websites, like QuickBooks Online (QBO), to act weirdly. Let's try logging into your QBO account using a private browser.
To use a private browser, here's how:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + P
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
Once logged in, go back and try to create an invoice again to double-check. For more guidance, feel free to read this article for more details: Create an Invoice.