With the new invoice layout, you can combine letters and numbers as your invoice number by modifying the field manually.
I've added this screenshot as your visual guide:
If you're still unable to modify the field, we can perform some troubleshooting steps to isolate the issue. To begin with, let's log into your account through an incognito window. You can use these keyboard keys to open one:
Google Chrome: Ctrl + Shift + N
Microsoft Edge & Mozilla Firefox: Ctrl + Shift + P
Safari: Control + Option + P
If it fixes the issue, you can continue creating the invoice. Then, you may go back to a regular browser and clear its cache. Piled cache can also be the reason for some unexpected issues on the opened pages.
You can also switch to a different browser like Google Chrome, Mozilla Firefox, Microsoft Edge, or Safari.