In order for EmpowerID to authenticate to your SharePoint tenant, you need to add to the key vault of the EmpowerID tenant the secret created when you registered the service principal used to authenticate the SharePoint app service.
Add the secret
Navigate to the key vault in the EmpowerID tenant.
Create a secret named SPOAppServiceAuthSecret and give it the value of the secret you created when you registered the service principal used to authenticate the SharePoint app service.