The type or namespace name 'UI' does not exist in the namespace 'UnityEngine'

So i came back to developing after a year break and my old project wont compile. I using version 5.5.4p4 and my project is from early 2018. I keep getting this error, “The type or namespace name ‘UI’ does not exist in the namespace ‘UnityEngine’”. I did some digging and already found out a lot of people reporting this problem, but none of the solutions works for me:

  1. Reimport all assets — no luck
  2. Delete Library folder and restart — no luck either
  3. Fixing reference on IDE — it fixed the IDE (Visual Studio) as in it dont report errors this error, but Unity it self keeps nagging about it. Also, something (Unity probably) keeps removing this reference.
  4. Import Ugui package — I dont know how.
  5. Add the dependency package to asmdef – Dont know how also.

Any tips? How to do itens 4 and 5? Or other solution entirely?

Unity UI came in Unity 4.6. You are using a version too old.