Are you allowed to plan an ATA in sealed if you have the points for it? If not why?

Sort of. Current new sets (IIM, SLOSH) have ATA's printed on the cards (Wrecking Crew, Hammer Industries, Legion Founder). If you pull figures that have these printed on them, then you can use the ATA. You can not bring some other past ATA (unless they start to print them on new figure cards)

Usually no, which is because you can only play what you pull out of the box in sealed. There are, however, some ATA's which come printed on the character cards. Wrecking crew being an example. Those are legal to play in sealed because they are being pulled along with your characters.

Like Puuka said, you can use the ones that come in the new sealed. Reason being, it's sealed only elements sealed in the package are allowed.

Ok kool, I was wondering with the war of light coming up soon, using some lantern ATAs. Thank you guys for responding so quickly! It's greatly appreciated.

From the Comprehensive Tournament Rules, there is one exception:

I am guilty of the same misrepresentation, my bad.