Apparently, the "if able" clause on this card is intended to prevent it from fizzling as easily as similar effects like Smite the Wicked. At least, that's what I glean from this response from FFG:

Q: "Hello! I have a question about Arkham Horror: The Card Game. Hank Samson's signature weakness, "Where's Pa?" reads: "Revelation - Discard cards from the top of the encounter deck until an enemy is discarded. Attach 'Where's Pa?' to that enemy and spawn it at a connecting location, if able."

I am wondering what happens if there is no connecting location, for example if I’m in the Study in the first part of The Gathering or in Rainy Streets in the first part of Riddles and Rain. The “if able” clause is attached to a sentence with two instructions (attach the card and spawn the enemy), so what if I’m not able? Thank you."

A: "Thank you for your interest in Arkham Horror: The Card Game. To answer your question(s): If there’s no connecting location when you draw “Where’s Pa?”, you will spawn the enemy at your location instead. Feel free to reach out to us if any more questions arise!"

Note that the answer says "your location," and not "following its typical spawn instruction." Maybe an oversight.

Wanted to make sure I got something right here, since this can be attached to an elite and non-elite enemies or enemies that cannot be defeated, Does this mean in those cases you are just out of luck and are doomed to take a horror damage for the rest of the game?

The first things that come to mind is Suspicious Orderly or the Hunting Horror. Is there anyone more common enemies that would be included? If so that'd really be rough since it seems the only way you can discard it is by killing the particular enemy. If you get it with the concealed cards in Scarlet Keys that's also really rough.

Edit: Another one comes to mind is Harbinger of Valusia, since when its set asside it keeps its attached damage, and possibly attached cards? In which case you'd keep the horror damage indefinitely.