Problem

For some reason Blender doesn't always have access to the numpy python library on the various Linux distributions. Not sure if it just comes default on python libraries for main OS's or what. When installing TrueTERRAIN (and many other addons) this error will popup.

Solution

First, ensure you have the most up to date version of Blender (or at least up to date version of the LTS).

We would recommend starting Blender from the terminal in order to see any issues along the way.

Next open up Blender and set an area to the Text Editor

Paste the following script into said text editor

import sys
import subprocess

exe = sys.executable
subprocess.call([exe, "-m", "ensurepip"])
subprocess.call([exe, "-m", "pip", "install", "--upgrade", "pip", "setuptools", "wheel"])
test = subprocess.call([exe, "-m", "pip", "install", "--user", "--upgrade", "numpy"])

if test == 0:
    print("Success")
else:
    print("Failure: Check terminal to know why")

And hit the Run button (or Alt + P alternatively)

In the terminal you can watch it install numpy and see if any errors occur