Why is Sucker Punch incompatible with Shiftry on Showdown?

Question

Pokemon Showdown won't let me battle my Shiftry because Sucker Punch is incompatable with another move or it's ability.

Here's my set:

ArtfulDodger (Shiftry) @ Focus Sash
Ability: Pickpocket
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Sucker Punch
- Foul Play
- Leaf Blade
- Swords Dance

I go to battle in Anything Goes and Showdown comes up with this:

Your team was rejected for the following reasons:

- ArtfulDodger (Shiftry) can't learn Sucker Punch because it's incompatible with its ability or another move.

I'm very confused, can anyone suggest why this is happening and suggest ways of getting round it?

Answer 1

It's because Shiftry can only learn Sucker Punch through move tutor in Gen 4. Since it cannot breed this move down and Hidden Abilities were not introduced until Gen 5, a Pickpocket Shiftry cannot know Sucker Punch as well.

Source

Answer 2

Sucker Punch was a move Shiftry could only learn in Generation 4 via move tutor. The first problem here is that Foul Play, an egg move, was introduced in Generation 5. The second problem here is that Pickpocket is also a Generation 5 ability. It would have been impossible to get a Shiftry with both Sucker Punch and Foul Play/Pickpocket since the move wouldnt pass down.

TL;DR: Get rid of Sucker Punch or Foul Play and Pickpocket and it should work because its an impossible set that can't be ingame without hacks.

Sources: Shiftry Parent Page, Shiftry Page, Smogon (shows that Pickpocket wasnt in gen 4), Foul Play page