I have been having trouble using a Hydreigon in online battles. I transferred it from my Ultra Sun game. It’s move set included Draco Meteor and Flash Cannon. Was trying to confirm if this would be the reason I am unable to use it?
This would not be the reason you can’t use it. Both moves are available. Draco Meteor still has the tutor, this time near the hot springs in Circhester. Flash Cannon is a TR. The reason you can’t use it is because only a Pokémon obtained in Galar is allowed to participate.
Source: https://www.serebii.net/swordshield/movetutors.shtml, https://www.serebii.net/attackdex-swsh/flashcannon.shtml, and experience.