Thank you for post!
I understand that you are unable to test the application and you are getting error message stating unable to test SAML SSO.
If you sign in successfully and then see an error on the application's page. This error occurs when Microsoft Entra ID issued a token to the application, but the application doesn't accept the response.
To resolve this issue please follow the article: Resolve a sign-in error on the application page
For more troubleshooting Debug SAML-based single sign-on to applications
Hope this helps. Please Do let us know if you any further queries.
Thanks,
Akhilesh.
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