Dependencies are managed by NuGetForUnity.

Before launching Unity, you need to run the following command in this directory to restore the dependent libraries:

dotnet tool restore

dotnet nugetforunity restore

Also, you need to execute the dotnet build command in the root directory where the .sln file is located; since the MessagePack library for development is referenced from bin\MessagePack\Debug\netstandard2.1 and bin\MessagePack.SourceGenerator\Debug\netstandard2.0, you need to generate the DLL with a debug build beforehand.

Also refer to bin\...\Debug\...\ directory contains package.json. This is a file needed for referencing on Unity and should not be deleted.