It's because itemurl is not a direct link to the file, but the full URL to view the post on tenor.com. You should use the media key instead. It's an array of dictionaries with the format (like gif, mp4, webm, etc.) as the key and a media object (with keys like size, duration, and url) as the value. It means you can get the URL of the gif image by using results[<random index>].media[0].gif.url.

I've updated your code, it works as expected now:

require('dotenv').config();
const Discord = require('discord.js');
const fetch = require('node-fetch');
module.exports = {
  name: 'tenor',
  category: 'misc',
  aliases: ['gif', 't'],
  async execute(message) {
      let url = `https://g.tenor.com/v1/search?q=Anime_Kiss&key=${process.env.TENOR}&limit=20`;
      try {
        let response = await fetch(url);
        let { results } = await response.json();
        let randomResult = results[Math.floor(Math.random() * results.length)];
        let { gif } = randomResult.media[0];
        let embed = new Discord.MessageEmbed().setImage(gif.url);

        message.channel.send({ embeds: [embed] });
      } catch (err) {
        console.log(err);
        message.channel.send('Oops, there was an error');
      }
  },
};