Go to the config in the file manager and find PermissionsEx then go to config.yml and change allow-ops=false to true save it then restart the server
once that is set. join your server and open chat box by pressing T on your keyboard and type this to get access to /pex after you set allow-ops to false again.
/pex user TacticalModz add permissions.*
Instead replace my name from TacticalModz to your minecraft name