I've taken the liberty of adding some checks that your faulty script is lacking:

on $*:text:/^!(add|del)item|(pickpocket)( |$)/iS:#:{
  if ($nick == your nick here) {
    var %file = pickpocket.txt, %item = $2-
    if (!%item) {
      .notice $nick please specify an item to add or delete:
      .notice $nick !additem <Items Here>
      .notice $nick !delitem <Items Here>
      .notice $nick !pickpocket <NickHere>
      return
    }
    if ($regml(1) == add) {
      if (!$read(%file,nw,$+(*,%item,*))) {
        write %file %item
        .msg # %item has been added to the pickpocket game.
      }
      else { 
        .msg # $qt(%item) already added! Please add another items. 
      }
    }
    elseif ($regml(1) == del) {
      if ($read(%file,nw,$+(*,%item,*))) {
        write $+(-dl,$readn) %file
        .msg # %item has been deleted from the pickpocket game.
      }
      else { 
        .msg # $qt(%item) is either removed or doesn't exist! 
      }
    }
    else {
      if (!$2) { 
        .msg # You need someone to pickpocket. 
      }
      elseif ($2 ison #) {
        .timer 1 1 .msg # $+($chr(2),$me walks up to $2 silently)
        .timer 1 2 .msg # $me reaches in $2 $+ 's pocket and finds
        .timer 1 3 .msg # $read(%file)
      }
      else { 
        .msg #  $+ $2 is not on # 
      }
    }
  }
}