Vets as a general rule will not spay a bitch while she is in season, they like to wait until things are back to normal, this is usually 3 months after the season or midway between cycles when all her bits and pieces are back to normal size and blood vessels aren't so engorged, my youngest bitch has just been spayed and she was done 2 months after her season as she was having seasons every 4 months.
Agree with the above, they can technically but don't like to, but they can give an injection called Delvosterone which should take her out of season. Speak to your vet if there is a genuine reason for wanting this done.