I have the exact same Problem: installed Mattermost 5.0.0 (Ubuntu 16.04) with MySQL 5.7 and I can create a first account but after that nothing happens - I can’t create a Team and got the same message as the poster above.
I tried to create a Team with the command line tool > worked, but then I tried to give adminstrator rights to my initially created account and nothing happens. Ist still a user. What can I do?
I used the following commands:
./mattermost team create --name myteamname --display_name “my team”
./mattermost team add myteamname myusername
./mattermost roles system_admin myusername