Why are some users undefined?

Question

JavaScript

Hello. My Discord bot doesn't see some users. It writes undefined
to the console . On some users, it collects information in the console, and on some it writes: undefined .
How to fix? please tell
me discord js v12

firstNumber = args[1];
  if (!firstNumber) return message.reply("Укажи ID");
      const Guild1 = robot.guilds.cache.get('516357819864186881')
            const user = Guild1.members.cache.get(firstNumber);
            console.log(user)