"Sorry, there's been a disconnect"

@Greg Burton
your URL definitely is incorrect if that's all you put in the add-in URL.

If you did specify the appId parameter, then the question is if you got the appId correctly
https://learn.microsoft.com/en-us/power-apps/maker/canvas-apps/embed-apps-dev#get-the-app-id

As you will want to have a title for your tile, you will also want specific the title parameter, making sure it is URL encoded ("My PowerApp" as title would need &title=My%20PowerApp appended to your URL.

If you have difficult in building the URL, you can try this:

@Greg Burton the URL builder is SOOO helpful!