In a terminal, with the board plugged in, enter.
ls -l /dev/ttyACM*, this will show the group that has access to the port.
Example: crw-rw-r-- 1 root dialout ... /dev/ttyACM0, root is owner, dialout is the group with access.
Entering the command, groups, in a terminal will show the groups you belong to.
sudo adduser YourUserName GroupToJoin Will fix things if you need to join a group, logout and login after you run adduser.
modem-manager can also be a problem -- most people just remove the package.