As a quick follow-up for anyone else stuck with this error... Flexera Support gave me this installation flag to skip Service Account validation: AppPortalSetup_2017_R1.exe /vVALIDATE_SERVICE_ACCOUNT=0 /vIS_NET_API_NET_VALID_USER=1

Make extra sure your login credentials are accurate because it's (obviously) not going to validate the settings against Active Directory.

While this resolved my issue I'm not yet entirely sure why it wasn't able to validate the credentials. I'm waiting for a response as the the requirements for that step to ensure my environment can support it.