You attempt to install Office Web App Server 2013 on a machine running Windows Server 2012R2 that has had all its windows updates applied and get the following error:
Expert TIP: This issue has come up before and there are a few blog sites that refer to it however the recommended fix has become outdated. The workaround involves uninstalling a patch so I will show you how to solve this yourself as over time even the example I show you will become dated.
Assuming you have followed the pre-requisites for installing the Office Web Apps Server and have installed .NET 45, the issue is that a newer version of .NET 4.5 has been installed due to server patching. This effectively blocks the installation of Office Web Apps Server and results in the error above.
Find out what version of .NET is installed on your server
In this case I dumped the version of .NET installed on the server using the following cmd:
Get-ItemProperty -Path “HKLM:SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full” | Format-List Release
This shows that release 461814 is installed. Some searching shows that this is .NET 4.7.2 – see https://docs.microsoft.com/en-us/dotnet/framework/migration-guide/how-to-determine-which-versions-are-installed#net_b for full list of versioning.
This means we need to uninstall .NET 4.7.2 before we can install Office Web Apps Server.
Some further searching shows what KB corresponds with .NET 4.7.2.
In this case its KB4054566.
Uninstalling the .NET patch
So we must uninstall that patch, reboot the machine and we should be good to go to install Office Web Apps Server.
Goto Add/Remove programs and select uninstall an update. Search for KB4054566 and uninstall it and reboot the server
After the restart, if you run the same cmdlet you will see the version is now 378675 which corresponds to .NET 4.5.1.
You will now be able to install Office Web Apps Server!