Let's check if you can edit or create invoices using a private browser. It is our first troubleshooting step in verifying if it's a browser issue or not. Let me guide you how.
In your Chrome, press CTRL+SHIFT+N. If you're using Mozilla Firefox, press CTRL+SHIFT+N.
Sign in to your account.
Go to Create (+), then Invoice.
Enter invoice information.
Click Save and Close.
If it's working fine, go back to your regular browser and clear your browser's cache. Your browser tends to hold onto information, and over time it could cause problems with logging in or bringing up websites. Thus, deleting your browser's cache fixes them.