If you are only using "Teachme Biz" added from the "Gallery" in Entra ID (formerly Azure AD), you may not be able to click "Get started".
Example 1: Added from the Gallery
Example 2: Added from the Gallery
Please check whether the issue is resolved by separately adding "Teachme Biz" via "Create your own application".
※ There is no need to Delete the app that has already been added.
Please refer to the online manual below for the steps to add an app via "Create your own application".
▼SSO via Entra ID (formerly Azure AD) (Web Browser)
https://teachme.jp/8/manuals/51167291