When I want to import a journal terms list on an existing library that already has references in it, I usually open the list, select all the terms (cmd+A) and click on “Delete term”. This is useful if you want a fresh start for this list.
It used to work on both PC and MAC. Since X9, this option only works on PC. Note that I can delete a single term at a time. See the attachments.
Also, I know that there is a workaround with deleting the list and recreating a new journal list.
I have seen this issue on multiple Mac computers since I train students with EndNote.