Hi Francis,
If you could provide more details, i can try to help you. In the meantime do read about the latest update on AD B2C support.
Azure Active Directory B2C is deprecating login.microsoftonline.com On 4th December 2020, the login.microsoftonline.com endpoint will be deprecated for accessing B2C tenants. Existing tenants will need to move onto B2Clogin.com before that date and all functionality will remain the same on the B2Clogin.com endpoint. See how to migrate your applications and web APIs to use B2Clogin.com.
New tenants created after this announcement will no longer be able to accept requests from login.microsoftonline.com. This does not impact Azure Active Directory (Azure AD) tenants and is only for Azure Active Directory B2C tenants. Ref: https://azure.microsoft.com/en-in/updates/b2c-deprecate-msol/