I’m using Unity 2021.1.15fl and the terrain tools are not in the packet manager anymore, even with the previews option turned on. I found a solution online:
Inside your Project window, right click Packages, click Show in Explorer, open the Packages folder, open the manifest file, add this “com.unity.terrain-tools”: “3.0.1-preview”, save the file, and go back to Unity. Unity will automatically import the packages. You will need to go back to Package Manager and update to the newest version “Version 3.0.2-preview.3 - November 09, 2020”.