Receiving Cannot POST error when using IdP-Initiated SSO Flow

I’m unsure how this part of the workflow is supposed to be configured and/or work.

We’ve provided our customer with the following ACS URL: https://[TENANT][CONNECTION-NAME]. When executing a SP-Initiated flow I’ve found that the implementation works as expected; however, the customer wants to launch the application from their own portal.

This takes them to the following URL: with a white screen that says Cannot POST callback.

I’ve never seen this before-- any ideas?

Changed the response type over to OpenID, set the scopes, and now it works fine.

FYI for anyone that needs it.