i just test the code no prob with nicklist may be u have something like nicklust.dll or another thing can do this prob
but i realized that u can check if nick exit or not in menu then u dont have to check and send an error msg that nick is already in accept nicks or other then in menu if this nick is in accepted nicks that will show del user or else add user

alias AddNick {
  .write AcceptedNicks.txt $address($1,2)
  echo -a 7 $+ $1 is now added to Accepted Nicks.
}
alias DelNick {
  .write -dl $readn AcceptedNicks.txt
  echo -a 7 $+ $1 is deleted from Accepted Nicks.
}
menu nicklist,query {
  PM Blocker
  .$iif($read(acceptednicks.txt,$address($1,2)),Del,Add) User:$iif($read(acceptednicks.txt,$address($1,2)),delnick,addnick) $$1
}