That message said ability or another move. The second one is much more likely. this is because in game, some times you cannot gain certain egg moves if trying to get more than one by breeding because the male cannot learn both moves, so you would have to choose either move but not both. To make sure they go with the actual games, Showdown have taken note of these moves, and made it so you cannot have both of these moves. Another thing could be that with event Pokemon, or certain Dream world Pokemon, you need certain natures or such criteria to fill before allowing certain moves , so that could possibly be the case when using it's DW ability, though it is unlikely.
This happens regularly, so just change the ability and/or moves a bit until it works.