If you are using only "Teachme Biz" added from the "Gallery" in Entra ID (formerly Azure AD), you may not be able to press the "Start" button.
Example 1 of adding from Gallery
Example 2 of adding from Gallery
You can try adding "Teachme Biz" separately from "Create your own application" to see if this resolves the issue.
*You do not need to delete the application you have already added.
Please refer to the following online manual for the steps to add an application from "Create your own application".
▼Single Sign-On Setup with Entra ID (formerly Azure AD) - Web Browser Version
https://teachme.jp/8/manuals/42394855