If you're referring to creating invoices in QuickBooks Online (QBO), try signing into your account using a private browser (incognito). This will help us check if the browser is already full of cache and cookies, causing this odd behavior.
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Once logged in, try to create a test invoice to double-check. Here's how:
Go to the + New option at the top left to create an Invoice.
Enter the necessary information in the desired fields.
Hit Save and close.
If it's successful, return to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.
To learn more about managing invoices in QBO, refer to these articles: