Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I have the same issue and have a ticket open with MSFT.
They're most recent response is:
Information reaching me now is that for Targeted release users, please locate the add tab (App) option in the top right corner of the channel. The entry point for adding tabs is shifting from the + button to the app icon in the top right of the header, as shown in the screenshot below.
So there is some kind of addin called "Channel Tab Group" which did let me add a tab.
Typical Microsoft taking something that is intuitive and putting it behind more clicks.