I had a FMS 14 running under MacOs Sierra.
I stop FMS 14 with the console and copy my files.fmp12 on the desktop.
I uninstalled FMS14 and installed FMS 15 Trial.
When installation done I get the message which ask me if I want to continue to start the Deployment assistant.
I click continue.
After 1 minute Safari shows me the following message:
The Database Server is not available. To edit server deployment, please start the Database Server first.
When I try to launch the console (http://localhost:16001/admin-console/) of course it does not work -> normal.
I try on a old Macintosh available running El Captain and I get exactly the same issue.
Same problem after restart.
The purpose is to try FMS15 before buying the update from FMS14.
You need to completely remove or rename the "filemaker server" folder that is left behind after uninstalling FMS14, then followed by a reboot.