Can't add script component because the script class cannot be found?

If you still have the old copy of the project, upgrade the Unity project to Unity 2017 first then to 2018.2.2f1.

Here are the few possible reasons you may get this error(Ordered from very likely)

1.Script name does not match class name.

If script name is called MyClass, the class name must be MyClass. This is also case-sensitive. Double check to make sure that this is not the issue. To make sure that's not the issue, copy the class name and paste it as the script name to make sure that this is not the issue.

Note that if you have have multiple classes in one script, the class name that should match with the script name is the class that derives from MonoBehaviour.

2.There is an error in your script. Since this is an upgrade, there is a chance you're using an API that is now deprecated and removed. Open your script with Visual Studio and see if there is an error there then fix it. There is usually a red line under a code that indicates there is an error.

3.Bad import with the Unity importer and its automatic upgrade script.

Things to try:

A.The first thing to do is restart the Unity Editor.

B.Right click on the Project Tab then click "Reimport All"

C.If there is still issue, the only left is deleting the problematic script and creating a new one. There is an easier way to do this if the script is attached to many GameObjects in your scene.

A.Open the script, copy its content into notepad.

B.From the Editor and on the Project tab right click on the script "CubeScript", select "Find References In Scene".

C.Unity will now only show all the GameObjects that has this script attached to them. Delete the old script. Create a new one then copy the content from the notepad to this new script. Now, you can just drag the new script to all the filtered GameObject in the scene. Do this for every script effected. This is a manual work but should fix your issues when completed.

Posting here for historic purposes and if someone from Google ends up here, I strictly had the same problem (I'm on Windows 10) and here's how I fixed it :

  • If you click on the Console, you should see a blank error. Don't worry, it's a Editor bug, and you can see the Editor Log by selecting the console's window dropdown and selecting Open Editor Log.

For me, the problem was that "Tools\Roslyn\csc" could not be found, something that Unity uses to compile C# files.

  • I opened C:\Program Files\Unity\Hub\Editor\2019.2.14f1\Editor\Data\Tools\RoslynScripts\unity_csc.bat and I added ".exe" to the line "%APPLICATION_CONTENTS%\Tools\Roslyn\csc" /shared %*

(This line should now be : "%APPLICATION_CONTENTS%\Tools\Roslyn\csc.exe" /shared %*)

Now it works perfectly.