Warning CS1685 as Error

I am getting warning CS1685 as Error (can not compile/run)

Details of the message reference two files:

1. C:\\ProgramFiles\Unity 5\Editor\Data\Mono\lib\mono\unity\System.Core.dll (Location of the symbol related to previous warning)
2. C:\Users\Roman\Documents\savit-unity\SourcesUnity\Savit.Unity\Assets\Plugins\x86_64\Microsoft.Scripting.ExtensionAttribute.dll (Location of the symbol related to previous warning)

And unhandled exception:

Last line of the error message is:

It should be only compiler warning so what does it Unity treat it like an error? More importantly what can I do about this error?

Thank you

 

ExtensionAttribute is implemented in both "System.Core.dll" and "Microsoft.Scripting.ExtensionAttribute.dll". You will need to remove one of the assembly references.

 

I have tried removing the file Microsoft.Scripting.ExtensionAttribute.dll but I get errors

 

Is it the same error or new errors?

 

If you're on an older Unity version change your platform in File > Build Settings to something other than Web Player.

 

I have stumbled upon this solution elsewhere, however my build target is set to PC

New errors

 

Why didn't you include the new errors with your reply..?

 

I am having the same problem implementing IronPython.

Going into the player settings API Compatibility Level to .NET 2.0, not the subset, fixed the problem.