You attempt to log into Symantec App Center from a mobile device that has had App Center access, but that has since been de-provisioned. You expect that your original user name won't work but are surprised that an alternative, known-good user name isn't working.
The App Center App (mobile client) is working as designed. All user names are dissociated from the device when the device is deprovisioned.
For more information, see Symantec App Center App - Enterprise support solutions.