I just did a clean install of Finale V25 onto my Mac recently upgraded to High Sierra. However, I couldn't get the Garritan sounds to work. Tried reinstalling these etc. 

Finally I though - maybe I should reboot. That solved it.

Mac users are known for leaving their computers on and simply putting them in sleep mode instead of shutting them off. I know someone who kept his on for over a decade with no issues. So for some of us, rebooting is a seldom thought of and usually avoided concept!