Unable to upgrade .NET framework

I am trying to upgrade the .NET framework to version 4 but encounter build error. I tried by removing C1.Web.C1Command.dll and again adding but the issue is not fixed.

 Could not load file or assembly 'C1.Web.C1Command, Version=1.0.20052.104, Culture=neutral, PublicKeyToken=96d8a77dc0c22f6b' 

or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)'