This issue could be faced because the required attribute has not been received in the SAML response. The attributes in the drop-down are added from the SAML response received from the IDP after Test Configuration is performed in the Configure IDP tab. To make sure that the required attribute is received in the SAML response, the attributes being sent from the IDP have to be configured. This can only be done from the admin console of the IDP being used.
If any assistance is needed for the configuration, please contact us at firstname.lastname@example.org.