Cannot find Incoming Webhook connector in Microsoft Teams

I had the same problem and I didn't remember that I (as a Tenant admin) disabled anything in the portal. "Unfortunatly" the screenshot is from the older portal, in the new Teams Admin check the following:

All available connectors were somehow blocked, so I removed everything from the "blocked apps" list:

Then I needed to sign out ("Quit" didn't refresh those settings) and sign in again and now I have access to all connectors.

As @wajeed-msft notes, this is happening because your administrator turned off access to external applications.

Assuming "Allow sideloading of external apps" is also turned off you'll get the list on the page you found above. If you scroll through that list, you will see "Incoming webhook" listed.