Discord Bot Can't Get Channel by Name

You should use the channnel id instead of it's name.

How to get the channel id of a channel:

1. Open up your Discord Settings

2. Go to Appearance

3. Tick Developer Mode (And close the Discord settings)

4. Right click on your desired channel

5. Now there's an option Copy ID to copy the channel id

Also checkout the discord.js documentation for (channel) collections

---

Furthermore your approach won't work because .get wants a channel id (see the linked documentation above). In case you REALLY want to get an channel by its name, use .find instead for that.
This is however a really bad idea in case your bot runs on more than one server since channel names can now occur multiple times.

You can also use

bot.channels.find("name","welcome").send("Welcome!")

I tried a lot with the same error, and that's how I fixed it. I used client as my Client().

client.channels.cache.get("18NUMBERIDHERE").send("Welcome!");