You can install Dotnet in Linux natively, that might be a better option than trying to use Mono. Try sudo apt search dotnet | grep installed to see if you already have some of it installed. I have all of the following packages installed locally on Pop!_OS aspnetcore-runtime-6.0 dotnet-apphost-pack-6.0 dotnet-host dotnet-hostfxr-6.0 dotnet-runtime-6.0 dotnet-sdk-6.0 dotnet-targeting-pack6.0 dotnet-templates-6.0 dotnet6. You probably don't need all of those installed, I think just sudo apt install dotnet6 will install all of the other packages as dependencies when you install it.
I'm not really familiar with Game Slug but you said you're using Lutris? Is it possible to try installing it through Steam or Heroic Games Launcher? I'm more familiar with those. Also, can you tell me which game it is that you're trying to install? That might help.