We faced same time of Issue and finally we found the reason is xconnect service url is not accessible by the windows service of Sitecore.
1. local url having sc91.sc sc91.xconnect doesnt work on some AD login and corporate machines.
Change the Installation Script with below Update
$Prefix = “sc91”
# The name for the XConnect service.
$XConnectSiteName = “sc91xconnect”
# The Sitecore site instance name.
$SitecoreSiteName = “sc91”
# Identity Server site name
$IdentityServerSiteName = “sc91identityserver”
Above will address the issue of Accessing Xconnect from the marketing automation windows service
Note: This has worked fine in our machines which uses AD login mechanism.
Sitecore 9.1 Installation