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.
Can you please check which version of Microsoft.Extensions.DependencyInjection.Abstractions is installed in your Server environment.
Sometimes the AssemblyVersion will be different from the AssemblyFileVersion; and the AssemblyVersion should be referenced and check web.config is referenced to which version of the assembly. Need to insatll that version or ...