The loops I use are not being recognized correctly.

[AttePatte]

on load:
  loop all players:
    loop all zombies in world of loop-player:
      add loop-zombie to {_zombies::*}
    add loop-player to {_players::*}

every 5 seconds:
  loop {_players::*}:
    set {_player} to loop-player
    loop {_zombies::*}:
      set {_zombie} to loop-zombie
      if {_zombies::*} is empty:
        continue
      if {_player} is not online:
        remove {_player} from {_players::*}
        remove {_zombie} from {_zombies::*}
        continue
      set {_path} to walkable path from location of {_zombie} to location of {_player}
      if {_path} is not set:
        continue
      set {_current} to first entry of {_path}
      remove loop-value-1 from {_path}
      while {_path} is not empty:
        teleport {_zombie} to {_current}
        set {_current} to first entry of {_path}
        remove loop-value-1 from {_path}
        wait 1 tick

on quit:
  set {_exited} to event-player
  remove {_exited} from {_players::*}
  loop {_zombies::*}:
    if loop-zombie is in world of event-player:
      remove loop-zombie from {_zombies::*}

on join:
  add event-player to {_players::*}
  loop all zombies in world of event-player:
    add loop-value to {_zombies::*}

It's possible that the Skript syntax or plugin version is different from what I'm familiar with.