Hi, I'm trying to watch a serie that I bought years ago on Apple TV and now the app do not allow me to Se the hole serie that I bought just the download episode, and when I try to buy the entire serie again on iTunes shows that is purchased

You only need to download episodes that you want available offline. You should be able to stream any of the episodes in the series when on Wi-Fi. The number of downloaded episodes will be limited by the disk space of your iPad. Remove downloads of episodes that you do not need to have available at this time.

Just click the preview frame for an episode to select it for playback.

See the Get More Episodes link to view more.

Select an episode to start playback.

Verify that you are online (Wi-Fi internet connected), to watch episodes that haven’t been downloaded in advance.

If on cellular, then verify that the TV app has the settings to allow that app to use cellular data (default is Off, I think).