Mac OS Mojave on older iMAC (circa 2012) no longer seems to support Amazon Music.
Looks like Big A has obsolesced the Amazon Music App on Mojave OS and possibly other older distributions. Someone in the forum suggested removing, then reloading Amazon Music. Not Possible! The Amazon app is no longer available in the Mojave Mac OS App Store. One more reason to ditch the old iMac, I guess.
iMac, OS X 10.11