earlier I thought it was something to do with Mono, as the 6.x version in nixpkgs was still at 6.0.
But, after trying 6.10 via this helpful PR and trying 5.20 direct from nixpkgs, they both get the same You must add a reference to assembly 'netstandard' error.
Removing mono doesn’t seem to be an option; isn’t it required if the project targets .NET Framework?