C# compiler throws Language Version (LangVersion) reference error “Invalid ‘nullable’ value: ‘Enable’ for C# 7.3”

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In my case, I ran into this problem with Visual Studio 2022 when I changed the target framework from .NET Standard 2.1 to .NET Standard 2.0. I solved my problem by removing <Nullable>enable</Nullable> in the .csproj file and restarting Visual Studio.

Original .csproj file:

<PropertyGroup>
  <TargetFramework>netstandard2.1</TargetFramework>
  <Nullable>enable</Nullable>
</PropertyGroup>

New .csproj file:

<PropertyGroup>
  <TargetFramework>netstandard2.0</TargetFramework>
</PropertyGroup>

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