db.Collection Is Not a Constructor

I’m having this issue where I’m trying to create a leveling system in Discord using discord.js and MongoDB. Every time I try to upload my data to the database, I keep getting the error message

db.Collection is not a constructor

I’ve followed the instructions from How do I save a new data to MongoDB? post on saving new data to MongoDB, but I’m still getting the same error.

Here is my code that I’ve been working on:

async execute(message) {
  if (!message.inGuild() || message.author.bot) return;

  const XPToGive = getRandomXP(5, 15);

  const query = {
    userId: message.author.id,
    guildId: message.guild.id,
  };

  try {
    const level = await Level.findOne(query).then(async (level) => {
      if (!level) {
        const newLevel = new level({
          userId: message.author.id,
          guildId: message.guild.id,
          xp: XPToGive,
        });

        await newLevel.save();

        return;
      }

      level.xp += XPToGive;

      if (level.xp > getLevelXP(level.level)) {
        level.xp = 0;
        level.level += 1;

        message.channel.send(
          `@${message.member} have been leveled up to **Level ${level.level}**`,
        );
      }

      await level.save().catch((error) => {
        console.error(`[ERROR] ${error}`);
        return;
      });
    });
  } catch (error) {
    console.error(`[ERROR] ${error}`);
  }
}

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I’m having trouble with using find() instead of findOne() in my code. It’s giving me more errors, and I really want my code to be error-free and able to save my data in the database.