Can't build error - dll is not allowed to be included or could not be found.

I'm trying to build and run my project but I keep getting errors. I believe the errors are related to the script I'm using called Uniducial, which references a program called reacTIVision to track markers with a camera. The error I'm getting is something to do with Unity Editor and DLLs. I'm way out of my depth here. Help!? This is the error:

ArgumentException: The Assembly UnityEditor is referenced by UniducialLibrary. But the dll is not allowed to be included or could not be found. UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/AssemblyHelper.cs:52) UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/AssemblyHelper.cs:55) UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/AssemblyHelper.cs:86) UnityEditor.HostView:OnGUI()

I also get these errors, but I think they're related to the above one:

Error building Player: Extracting referenced dlls failed. UnityEditor.HostView:OnGUI()

And:

Exception: Error building Player: Extracting referenced dlls failed. UnityEditor.BuildPlayerWindow.BuildPlayerWithDefaultSettings (Boolean askForBuildLocation, BuildOptions forceOptions) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/BuildPlayerWindow.cs:341) UnityEditor.BuildPlayerWindow.GUIBuildButtons (Boolean enableBuildButton, Boolean enableBuildAndRunButton, Boolean canInstallInBuildFolder) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/BuildPlayerWindow.cs:942) UnityEditor.BuildPlayerWindow.ShowBuildTargetSettings () (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/BuildPlayerWindow.cs:923) UnityEditor.BuildPlayerWindow.OnGUI () (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/BuildPlayerWindow.cs:697) System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation. System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/GUI/DockArea.cs:213) UnityEditor.HostView.Invoke (System.String methodName) (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/GUI/DockArea.cs:206) UnityEditor.HostView.OnGUI () (at C:/BuildAgent/work/6bc5f79e0a4296d6/Editor/Mono/GUI/DockArea.cs:107)

you can try: file/buildsettings/playersettings/othersettings change the .net2.0 subset to .net2.0 good luck.

In the end updating to a new version of some open source .NET code I was using sorted the problem out.

Turn off Mono, it turn out that some of my unused dll files was being used by Mono, so Unity can’t delete it. Hope it work out for you