Unable to get VSCode OmniSharp working with Unity

I've migrated a project from Windows over to Ubuntu and set it up to work with VSCode. However, it seems to have issues with the assembly files. Here is the error:

Spoiler: Click for error

Failed to load project file '/home/sc-player/Documents/Unity/Defense/Assembly-CSharp-Editor-firstpass.csproj'.
/home/sc-player/Documents/Unity/Defense/Assembly-CSharp-Editor-firstpass.csproj
/home/sc-player/.vscode/extensions/ms-vscode.csharp-1.17.1/.omnisharp/1.32.8/omnisharp/msbuild/15.0/Bin/Microsoft.Common.CurrentVersion.targets(1195,5): Error: The reference assemblies for framework ".NETFramework,Version=v4.7.1" were not found. To resolve this, install the SDK or Targeting Pack for this framework version or retarget your application to a version of the framework for which you have the SDK or Targeting Pack installed. Note that assemblies will be resolved from the Global Assembly Cache (GAC) and will be used in place of reference assemblies. Therefore your assembly may not be correctly targeted for the framework you intend.

I've done extensive reading and I cannot fix it, even temporarily, by changing the .NETFramework version. I've tried updating mono as well:

sudo apt-get install mono-complete

 

I'd guess that you don't have the required version. I'm not a linux pro, but when i installed unity i installed mono following this procedure https://www.mono-project.com/download/stable/#download-lin and that worked for me (i would guess that the base repository lags a bit behind).

 

As I understand it, Mono is a replacement for the .NET Framework, since the framework is only available on windows? If that is the case, how do the versions correspond? The reason I ask is because I can't find this information anywhere. Currently pre-compiling the newest version.

Edit: That seemed to do it. Thanks, I'm mostly just wondering now how I know this is the correct version

 

In terminal "mono --version" reveals the current mono version. I have 5.18.0.240 and newest seem to be 5.18.0.268.

 

So the newest version will always be the correct one?

 

Dunno, newest version should have most features that older mono is missing from .net.
"Sponsored by Microsoft, Mono is an open source implementation of Microsoft's .NET Framework based on the ECMA standards for C# and the Common Language Runtime" from https://www.mono-project.com/