Sometimes, when you add Flexbox4Unity to an existing project, Unity will generate this error in your console:
error CS0234: The type or namespace name ‘EventSystems’ does not exist in the namespace
This is caused by a small bug in Unity itself: they keep changing their internal UI classes in incompatible ways (in 2020 Unity is now onto the third complete rewrite for the Unity UI system, each time they give it a new name and make it incompatible with what they provided before).
Fortunately this is usually easy to fix – you just need to re-import Unity’s own UnityUI library (it’s unfortunate that they didn’t fix this themselves, and do the import automatically).
- In the Project view, open the Packages section
- Find the “Unity UI” folder
- Right click “Unity UI” folder and select “Re-import”
Re-importing should take 5-10 seconds, after which your compiler error should have disappeared.