[Skript of Mana, why will it detect the event?]

I use this code but it wont work...

on first join:
    set {maxmana.%player%} to 30
    set {mana.%player%} to {maxmana.%player%}

------------------------Down there it says error------------------------

every 50 ticks:
    loop all players:
        if {mana.%loop-player%} is greater than {maxmana.%player%}:
            set {mana.%player%} to {maxmana.%player%}
        else:
            add 3 to {mana.%loop-player%}