Why can’t I find Apple TV in my devices

If you are referring to Find My, then an Apple TV box isn’t supposed to show there.

An Apple TV box doesn’t know where it is.

Yup, your original post isn't quite clear. Did you purchase:

• Apple TV+ THE APP or a
• Physical Apple TV box

Once we know what you bought and what you have done to-date then we can begin pointing you in the right direction. Until then we are scratching our collective heads.

Do you mean it’s not showing up in the devices signed in with your Apple ID? Have you activated it? Until you do that and sign in with your Apple ID / iCloud Account, it won’t. You can view your devices on an iPhone in Settings- Your name . If you added it to HomeKit, it needed to be assigned to a room and might just be called “Livingroom“, for example. Check your Apple ID device list to find where you're signed in - Apple Support

Ypu can use the serial number to see your warranty: Check Your Service and Support Coverage - Apple Support

Otherwise, as stated above please clarify

On your Apple TV go to General - About, to get the serial number then go here: Check Your Service and Support Coverage - Apple Support to verify you have Apple Care. When I look in subscriptions I only see it for my iPhone, not my iPad, HomePods or MacBook. ’m paying monthly on the iPhone (so it’s a subscription) and the others were yearly or an up front lump sum.

As for it not being in your device list, are you signed into the same Apple ID you’re using for your main default account?

• Try signing out, on the Apple tv in Users & Accounts - sign out of iCloud, restart your Apple TV , then sign back in. Then see if it’s in your device list. This might not be instantaneous where it show up.

If you’re in N.America you can use this number to call Apple:  1-800-MY-APPLE and ask them to check it for you.

Apple tv is an App or a physical Apple TV box. The App should be on most updated Apple devices, Smart TV’s or cable boxes

If you mean Apple TV + see: How to watch Apple TV+, MLS Season Pass, and more - Apple Support