PLEASE READ - This forum is not for uGUI (or Unity UI), or IMGUI

This forum is for UI Toolkit (formally named UIElements), which was introduced in Unity 2019.1 as new UI API that is currently only available for the Editor.
Docs: https://docs.unity3d.com/Manual/UIElements.html
Namespaces:

UnityEngine.UIElements

and

UnityEditor.UIElements

For uGUI, or Unity UI - current way to create in-game UI:
Forum: https://forum.unity.com/forums/unity-ui-textmesh-pro.60/
Docs: https://docs.unity3d.com/Manual/UISystem.html
Namespaces:

UnityEngine.UI

and

UnityEditor.UI

For IMGUI, or OnGUI() - pre-2019.1 way to create Editor Extensions:
Forum: https://forum.unity.com/forums/extensions-ongui.25/
Docs: https://docs.unity3d.com/Manual/GUIScriptingGuide.html
Namespaces:

UnityEngine

and

UnityEditor