Failure of building ue4 from source code: error MSB3644: The reference assemblies for .NETFramework,Version=v4.6.2 were not found

You need to clean the UnrealBuildTool folder ( \Engine\Source\Programs\UnrealBuildTool ) of any temporary data (bin/obj/.vs), I had a previous 5.0 build and it was interfering with a 4.27 switch

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