@Lee , You have posted the question on azure-webapps, I have added additional tags to receive insights from the targetted SMEs/audience.
Could you please let us know if you're leveraging Azure App Service WebApps or referring to some other app?
If your application has been setup to use Azure AD for identity management using SAML-based SSO.
Cause: The AssertionConsumerServiceURL value in the SAML request doesn't match the Reply URL value or pattern configured in Azure AD.
The AssertionConsumerServiceURL value in the SAML request is the URL you see in the error.
To fix the issue, try these steps:
Ensure that the AssertionConsumerServiceURL value in the SAML request matches the Reply URL value configured in Azure AD.
Verify or update the value in the Reply URL textbox to match the AssertionConsumerServiceURL value in the SAML request.
See this doc - https://learn.microsoft.com/troubleshoot/azure/active-directory/error-code-aadsts50011-reply-url-mismatch