
Hello, this usually happens because multi-factor authentication (MFA) or security defaults are enabled in the tenant. When a user from an external tenant signs in, Microsoft Entra ID may require them to set up an additional verification method before they can access the app. Since you created a new tenant and invited your personal email, the sign-in flow is likely enforcing MFA registration. In many cases the easiest fix is simply to complete the MFA setup when prompted, and then the user will be able to access the Microsoft Power Pages site normally.
If the goal is to avoid MFA for testing, you could temporarily disable Security Defaults or adjust the Conditional Access policy for that user.
Best Regards,
Kaveri