I got a year free of Apple TV . Tried watching on my 4 years old Samsung tv and the app is not available; tried signing in with my tv browser but Apple page stops at sign in. I then tried my 4 y/o Sharp Romualdo tv and app not available.

Are you sure you’ve activated the free year? It’s not automatic.

Use the same Apple ID you activated the new device with

Here’s how to redeem it:

Turn on your new Apple device and sign in with your Apple ID

Open the Apple TV app. Make sure your device is running the latest version of iOS, iPadOS, tvOS, or macOS.

The offer should be presented immediately after launching the app. If not, go to the Apple TV+ channel page, signed in, where you’ll see the offer.

All the ways to watch Apple TV+ - Apple Support

https://support.apple.com/en-ca/guide/tvplus/welcome/web

Tap Enjoy 1 Year Free.

Offer must be claimed within 90 days from when you activated the new device 

Your new device must also be capable of running the latest iOS, iPadOS, tvOS, or macOS. Offer cannot be combined with other free trials or offers for Apple TV+.

Your model needs to support the TV App. If it does you do a firmware update and install the app

Instructions are in the links I provided

Smart TVs

Streaming devices