Question: I recently opened a project created in an older version of unity with a newer version, However I am getting an error saying 
Unknown API compatibility level: 6.
UnityEditorInternal.InternalEditorUtility:GetMonoIslands()
UnityEditor.SyncVS:SyncVisualStudioProjectIfItAlreadyExists()

Furthermore my game view is aqua green and my scene view is black. Please help!

[![enter image description here](https://i.stack.imgur.com/CP5Eg.png)](https://i.stack.imgur.com/CP5Eg.png)

Answer: 
please follow the next steps:

1. Go into your project's folder
2. Open `ProjectSettings` and open `ProjectSettings.asset` with a text
editor
3. Find `apiCompatibilityLevel` and change it from 6 to 2
Unity does not handle very well changing the editor version. 

You can read this post to have an official statement from an Unity Employee, where he explains how it handles backwards and forwards compatibility:

<https://forum.unity.com/threads/compatibility-across-unity-versions.556927/>

I suggest that you use the old version you used to develop the original game.

Downloaded the old editor here: <https://unity3d.com/get-unity/download/archive>

Regards.