After some more research, it turns out that a previous failed install was causing this problem.
The previous install had placed some files, but most likely not everything.
We were able to fix it by first uninstalling the failed installation, and then running the unattended install again.
AFAIK - SDL Tridion Sites 9.1 - you need to manually install the .Net framework 4.8 and .NET Core Runtime 2.2 manually prior to install.
Note: These pre-requisites not comes with SDL Tridion Sites installation media.