'This will not work on Pokémon from the Galar Region'
Yet it did not work, as I apparently need a Galar TM, since Flygon is in Sword and Shield, something that was not obvious. Also the error message should be more specific in *why* the Pokémon couldn't learn the move, such as telling you 'This Pokémon requires a Galar TM to learn this move.'
I'd also suggest adding a simple, single config entry to allow TMs to work on Galar Pokémon and Vice Versa, as the current system is very confusing.