It's nice to see you here in the Community, @sart7.
There are several ways on how to enter an invoice in QuickBooks Online. This can be done through the + New button, customer section, and in the general invoice list.
If you're unable to create an invoice with any of these options, I recommend accessing your account using a private window. This is to test if the problem is browser-related.
At times, the stored data can cause some odd issues and simply needs to be cleared from the browser.
Here are the different keyboard shortcuts to open a private browser: