I was playing last night and I tapped on a Pokémon to try and catch them and an error message popped up saying "The Pokémon is too far away." The Pokémon was within my radius, I wasn't driving, and my GPS signal was stable, so I don't know why it would say that. How far away is too far away to catch a Pokémon, or why else would this message appear?
The farthest away that you can catch pokémon is 40 meters away from you, but since it is not very accurate, but sometimes you may be able to catch pokemon from 60-100 meters since it is not very accurate. You can find more information about that at this link: https://www.reddit.com/r/pokemongo/comments/4qxb84/question_about_the_max_range_to_interact/
Hope this helps you! :)