Error Building Player because scripts had errors

Hi there,

I am new to unity and was just trying to get a webgl build for a project. This message has come up with the accompanying information

Error: System.Exception: 'UnityEngine.MonoBehaviour' was not found in any of the referenced assemblies
at us.UnityScriptCompilerFactoryModule.resolveType (System.Object assemblies, System.String typeName) [0x00000] in <filename unknown>:0
at us.UnityScriptCompilerFactory.FromCommandLineOptions (us.CommandLineOptions options) [0x00000] in <filename unknown>:0
at us.UsModule.compile (us.CommandLineOptions options) [0x00000] in <filename unknown>:0
at us.UsModule.runWithCommandLine (System.String[] commandLine) [0x00000] in <filename unknown>:0
at us.UsModule.Main (System.String[] argv) [0x00000] in <filename unknown>:0



The project runs absolutely fine in unity but will not build into WEBGL or any other format. Some guidance would be much appreciated. If you require any more information then please do not hesitate to ask.

 

Apologies just thought, I am building in Windows 7 using unity 5.6.5f1