The problem with Unity update

I have a project which is made by Unity 4.6.9. When I open it in Unity 5.4.2,the following error display in console.

Unhandled Exception: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded.

at (wrapper managed-to-native) System.Reflection.Assembly:GetTypes (bool)

at System.Reflection.Assembly.GetTypes () [0x00000] in :0

at Mono.CSharp.RootNamespace.ComputeNamespaces (System.Reflection.Assembly assembly, System.Type extensionType) [0x00000] in :0

at Mono.CSharp.RootNamespace.ComputeNamespace (Mono.CSharp.CompilerContext ctx, System.Type extensionType) [0x00000] in :0

at Mono.CSharp.GlobalRootNamespace.ComputeNamespaces (Mono.CSharp.CompilerContext ctx) [0x00000] in :0

at Mono.CSharp.Driver.LoadReferences () [0x00000] in :0

at Mono.CSharp.Driver.Compile () [0x00000] in :0

at Mono.CSharp.Driver.Main (System.String[] args) [0x00000] in :0

Missing method .ctor in assembly F:\105\Previous_Project\Fishing (1.17) last source\New Unity Project 2\Assets\Plugins\WP8\GoogleAds.dll, type System.Runtime.Versioning.TargetFrameworkAttribute The class System.Runtime.Versioning.TargetFrameworkAttribute could not be loaded, used in GoogleAds Can't find custom attr constructor image: F:\105\Previous_Project\Fishing (1.17) last source\New Unity Project 2\Assets\Plugins\WP8\GoogleAds.dll mtoken: 0x0a00001b

If anybody can fix this problem,please help me. Thank you!