Unable to Save External Authentication Method

Question

I am attempting to add a new EAM(External Authentication Method) to the Authentication Methods blade, and whenever I attempt to save I get an error:

I have triple checked and all of the fields are correct, the admin consent has been granted and confirmed on the Enterprise App. I know the links are correct as they were auto generated by the product and copy/pasted directly in. We need to move the app(Cisco Duo) from our Custom Controls over to EAM I believe to keep things working in the Authentication Methods. If this is untrue, please let me know.

We would of course prefer to continue using Custom Controls until we deprecate the app in the next 6 months. But if it will interfere with the migration from legacy Multifactor Authentication policies to Authentication Methods, then that work must be completed by Sept 30th (in accordance with MS guidance). Any help is greatly appreciated!