Troubleshooting

Error: The reference assemblies for framework ".NETFramework,Version=v4.7.1" were not found.

TL;DR:

• Go to https://aka.ms/msbuild/developerpacks and download .NET Framework v4.7.1.

Details:

• Unity includes its own, custom implementation of the .NET Standard 2.0 specifications that are included within the Unity distribution - i.e. C# code executed by Unity does NOT use the .NET Framework installed on your computer (I'm pretty sure), but rather, Unity itself contains everything that you need.
• "Unity .NET" (not a real term) corresponds to a particular version of .NET Framework.
  • You can think of .NET Framework as the "parent" of "Unity .NET"
• Other tools, like Rider, don't understand "Unity .NET" - they understand .NET Framework.
• Unity tells tools like Rider what version of .NET Framework to treat "Unity .NET" as by defining <TargetFrameworkVersion> in the Unity-generated .csproj files, e.g. Assembly-CSharp.csproj
• Rider (and probably other tools - it looks like VSCode in particular has this issue a lot) requires that you have the exact .NET Framework version installed on your computer that the .csproj files ask for.
• You might have the most up-to-date version of .NET Framework on your computer - for example:
  • On 5/8/2020, using Unity 2019.3.13f1;
  • The most recent .NET Framework version was 4.8 (released 2019-04-18)
  • Unity set <TargetFrameworkVersion> to 4.7.1 (released 2017-10-17)