The dog can skateboard should be accepted. In English we don't say ride a skateboard. Skateboard is a verb.
It's also a noun, and you can say ride a skateboard. Yours should be accepted though.
I grew up in a place with lots of rollerskating, skateboarding, and rollerblading (my favorite growing up). You can "ride" anything with a board (scooters too but those didn't have any cool factor past elementary school) but not things you strap onto your feet.
It failed to accept the answer however it showed the correct answer that is exactly the same. Maybe network issue