Failed to authenticate Staged users (Authentication Failed)

I tried to authenticate STAGED users through either /authn or Java SDK in Service app but I kept getting Authentication Failed despite providing correct credentials!

Is there a way to determine whether a STAGED user has provided a correct credentials?

Note: I have to get this information (user has a valid credentials) in order to proceed to next step which is activating user account.

A user needs to be moved too active before authentication can take place.