Unable to start Mattermost service in Ubuntu 15.10: getting error as "Failed to start mattermost.service: Unit mattermost.service failed to load: No such file or directory"

I just installed mattermost 5.0 on Ubuntu 16.04 LTS for the first time and when I try to start it (as directed in the install directions, I get:

/opt/mattermost > sudo -u mattermost ./bin/mattermost
unable to execute ./bin/mattermost: No such file or directory

The closest thing I’ve found indicated that this means the mattermost binary can’t find config.json, but I’ve been able to get it running on a different 16.04 server, so I have no idea what the issue is.