c# err: The reference assemblies for framework ".NETFramework,Version=v4.5" were not found

c# err: The reference assemblies for framework ".NETFramework,Version=v4.5" were not found
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Project 'ClassLibrary3' load finished with warnings: The reference assemblies for framework ".NETFramework,Version=v4.5" were not found. To resolve this, install the SDK or Targeting Pack
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