Hi Aaron Booterbaugh,
Have you checked the installer's checksum value against the one listed by Esri Inc?
In https://my.esri.com you should be able to find the checksum listed against each of the installation media (Portal for ArcGIS 10.8.1 is:
18feacad793bb3819fd7048588a7edb3
If support have said this upgrade has worked for other sites, it would likely be an issue with your particular media (maybe the download somehow corrupted) or with the environment that you are installing onto.
I have not yet upgraded my environments to 10.8.1 as I am waiting on other dependencies that are out of my control, but I know that when installing we need to run all installations by right clicking the executable and selecting "Run As Administrator"
If that doesn't work, would it be possible to talk with your IT team and have them temporarily disable any security/anti-virus on a separate test VM and install Portal for ArcGIS 10.8.1 on that machine?
Another thing to double check, is that the Service Account permissions on install and content directories - ensure the account has the ability to modify these 2 directories.