Symptoms

The onPrem SharePoint server was not configured for SSL or HTTPS. When the server was trying to reach the cloud it did so with an HTTP call and the Appit instance is/was expecting an HTTPS call and as such Appit could not be properly added.
 

Diagnoses

This was an environment that had not been configured for SSL and this is not supported by Appit.
 

Resolution

While we are still investigating mixed protocols of HTTP and HTTPS this issue is currently best resolved by configuring SSL for the onPrem server. After that was done the user could actively add Appit functionality to the various site collections.