[BD/SP] How come when I put the move Toxic on a lot of Pokemon, PKHeX flags it as an invalid move?

[Guest]

How come when I put the move Toxic on a lot of Pokemon, PKHeX flags it as an invalid move?

 

Has happened with many Pokemon but the one I'm particularly working on right now is Articuno.

[BlackShark]

Toxic is unavailable for Articuno in BDSP.

The only way for it to learn Toxic was a TM in Gen 1 - 7 which doesn't exist in BDSP and there's no way to transfer mons to these games yet.