Only the FM 15 product line is compatible with Sierra... so yes: roll back.
Thanks, but I’m not making any progress on the rollback. I made an El Capitan USB boot, but I can’t get Sierra to allow me to access it on booting, The Option key on startup does not work. I wondered if FM Server is interfering, but the only installation file that had an option for uninstalling FM Server was a recent ver 14 patch. I tried that, and manually deleted FM Server in the Library, but still no luck with the option key. Selecting Boot Disk in System Preferences doesn’t work apparently El Capitan is too old for Sierra to recognize as a bootable on the USB stick. A web search suggested using rERit, but it didn’t work either, its documentation doesn’t mention anything beyond Snow Leopard. Any suggestions?