Installing VSCode in Linux with Flatpack is not enough.
You’ll usually get a message that DotNET and Mono need to be installed, but with no further instruction. Here’s what you need to do, Open up a terminal and type these two commands:
flatpak install org.freedesktop.Sdk.Extension.dotnetflatpak install org.freedesktop.Sdk.Extension.mono6
Credit to SopyGG for this info.