Zacznijmy od tego, że kod skrócisz do 4 linijek (jak się postarasz to nawet do 2 lub 1)
loop {lvl::%player%::*}:
if loop-value = true:
set player's current inventory's slot loop-index to lime stained glass pane named "&aLevel %loop-index%!" with lore "&6Nacisnij aby zagrac!"
else:
set player's current inventory's slot loop-index to red stained glass pane named "&aLevel %loop-index%!" with lore "&6Nacisnij aby zagrac!"
Dodatkowo aby to zrobisz, możesz użyć eventu on join
on join:
loop 6 times:
{lvl::%player%::%loop-value%} is not set
set {lvl::%player%::%loop-value%} to true