My first add-in: API Error - Where have I gone wrong?

I wanted to create an add-in that, when clicked, triggers a Power Automate flow that would run a query and send the results to me via email.

I used the "Trigger a flow from a SKY Add-in button" template for the first two actions:

I used the schema from the template.

My application has been created and is visible on the "manage" tab of marketplace.

I was able to obtain my application ID and save and get the URL as instructed.

My application has been created and visible on the "manage" tab of marketplace.

The flow works perfectly in the copy where it is triggered manually.

I can see it on my homepage (see below, it is "Annual Funds Stats").

But when I click it, I get:

I have three theories:

1. there is an issue with the schema, and therefore the URL
2. for my application, I reallly struggled with what a redirect URI is and how to get one, and so used the one provided through the "Create an acknowledgement letter from an add-in button on the gift record" tutorial (https://global.consent.azure-apim.net/redirect) which was probably NOT the thing to do but I was at a loss.
3. I only gave my application "read only" permissions through scope - does it need more?

Of course…I could be wrong on all counts…🤷‍♀️