A shadow user is a user that is automatically created on the SAP BTP subaccount level when a user logs in to SAP Build Work Zone for the first time1.The shadow user is assigned the SAP BTP subaccount viewer role collection, which is required for accessing SAP Build Work Zone2. If a shadow user does not exist on the SAP BTP subaccount level, the user cannot login to SAP Build Work Zone. The other options are not valid reasons for the login issue, because:

SAP BTP global account viewer role collection is not required for accessing SAP Build Work Zone2.

Active default identity provider on SAP BTP subaccount level is a prerequisite for enabling SAP Build Work Zone3, but not a reason for the login issue.

SAP BTP subaccount viewer role collection is automatically assigned to the shadow user2, so it is not missing unless the shadow user does not exist.Reference:1: Shadow Users2: SAP BTP Subaccount Viewer Role Collection3: Enable SAP Build Work Zone