Problem

Some nodes are linked to blend files in the addon's folder. So if the addon's folder is moved, those nodes and therefore those blend files will be broken and/or not work.

Solution

Turn your outliner to Blender File.

You should see error/warning icons next to the libraries (either the ones in dropdown or the ones at the very bottom)

Right click and select Relocate in the menu that appears

Optional: In the file browser window that appears enable the cog (circled in green) and ensure Relative Path is off (underlined in blue)

Then look in your addons folder for the true_terrain folder and navigate to the blends directory (true_terrain\blends) and then find whatever blend file the library is wanting to look for (should already be in the bottom input bar)

Prevention

To prevent this from happening the best solution would be to make local all data before moving. To make all data local and not referencing other blend files go to the 3D Viewport's header menu and go to Object >> Relations >> Make Local... >> All . This will make all data (not just objects) local to the open blend file.