NVM, I found the solution for this particular trainer. You need to download and install the .NET Core 3.1 Desktop Runtime. Unfortunately the message is not very clear, when you select the version there many different versions to choose from SDK, ASP .Net, .Net Core etc.
Like previosly mentioned, you want the Desktop Runtime 3.1.
You can get it here: dotnet.microsoft.com/download/dotnet-core/thank-you/runtime-desktop-3.1.2-windows-x64-installer
hope this helps
I can confirm that it still works on bethesda after update 😍