Loop player not working

[2zie]

I tried making a script that will select a player, but it keeps saying 'cant understand condition/effect: loop all players' how can i fix this?\

the script:

on join:
    if player has permission "op":
        add 1 to {staffonline}
        loop all players
        if loop-player has permission "op":
            set {chosenstaff} to loop-player
    else:
        if player has permission "op":
            set {chosenstaff} to player