I am trying to follow this answer: Adding Firebase Admin SDK to Unity project
But I can't get it to work. I installed the nuget plugin for unity, I installed the firebase admin sdk package and I installed it's dependencies. I get a bunch of errors that I don't know how to fix, and I am confused even though, the person in the thread says "just follow the instructions", it doesn't work.
It looks like you're missing some dependencies (System.Collections.Immutable). Since this is provided by the system, my recommended course of action would be to open Player Settings:
And change the "Api Compatibility Level*" under "Other Settings":
I think .NET 4.x might help (and .NET Standard 2.0 is default, and it looks like that's what's in the error log), but whatever you currently have selected try the opposite.
A final thing to try is to run this on a Windows machine if you're currently on Mac/Linux. Under some conditions, it will use the system .NET rather than the version of Mono that ships with Unity (I believe this is only the case for a UWP release build, but maybe it applies to the editor).