My bot keeps saying "User not found" though the user is on my server

this is my code. I been only able to mute some bots and myself and a few users but not all, I'm not sure if I'm doing anything wrong but any help would be appreciated :)

else if (isValidCommand(message, 'mute')) {
    message.delete()
    if (!message.member.hasPermission(['KICK_MEMBERS', 'BAN_MEMBERS'])){
      message.channel.send("You don't have permission to use this command.");
    }
    else {
      let memberId = message.content.substring(message.content.indexOf(' ') + 1);
      let member = message.guild.members.cache.get(memberId);
      if (member) {
        if (member.hasPermission(['KICK_MEMBERS', 'BAN_MEMBERS']) && !message.member.hasPermission('ADMINISTRATOR')) {
          message.channel.send("You cannot mute that person!");
        }
        else {
          let mutedRole = message.guild.roles.cache.get('956241047753744414');
          if (mutedRole) {
            member.roles.add(mutedRole);
            message.channel.send("User was muted.");
          }
          else {
            message.channel.send("Muted role not found.");
          }
        }
      }
      else {
        message.channel.send("Member not found.");
      }
    }
  }