Problem:

When trying to use the Terrain Model>Create from File command, I receive the message "Import Failed"

Solution:

This message is received when the surface DTM has the "Lock Triangulation" or "Extended Data Checks" toggles turned ON in the Surface Properties dialog. To workaround the failure, turn both the "Lock Triangulation" and the "Extended Data Checks" toggles OFF and save the DTM file.  Then run the Create from File command to import the DTM.