You are trying to get from the cache a Guild not a Channel, or more specifically a TextChannel, that's why what you try is not working, you should try to change the code to something like this and it should work.

const channel = client.channels.cache.get('Channel Id');
channel.send({ content: 'This is a message' });

Also, as something extra, I would recommend using the fetch method for these things, since it checks the bot's cache and in the case of not finding the requested object it sends a request to the api, but it depends on the final use if you need it or not.

const channel = await client.channels.fetch('Channel Id');
channel.send({ content: 'This is a message' });

There are three ways of sending message to a specific channel.

1. Using a fetch method

const channel = await <client>.channels.fetch('channelID')
channel.send({content: "Example Message"})

2. Using a get method

const channel = await <client>.channels.cache.get('channelID')
channel.send({content: "Example Message"})

3. Using a find method

a.

const channel = await <client>.channels.cache.find(channel => channel.id === "channelID")
channel.send({content: "Example Message"})

b.

const channel = await <client>.channels.cache.find(channel => channel.name === "channelName")
channel.send({content: "Example Message})

Your current code line is getting the GUILD_ID not the CHANNEL_ID.