At the beach : somewhere on a shorefront location (town, boardwalk, shop, restaurant, in the water...)
On the beach : actually on the sand that adjoins the water
Is that your line of thinking as to why on is not accepted?
Tried "on" and it is accepted
It must have been fixed. Thanks!
Thanks to you for spotting it :)
"She plays at the beach with you" should also be accepted
On was not accepted as correct.