Which form of volleyball does NOT exist?

There is no such form of volleyball as skateboard volleyball.

Volleyball is a team sport in which two teams of six players are separated by a net. Each team tries to score points by grounding a ball on the other team's court under organized rules. It has been a part of the official program of the Summer Olympic Games since Tokyo 1964.

Snow volleyball is a winter team sport played by two teams of three players on a snow court divided by a net. The objective of each team is to score points by sending a ball over the net so as to ground it on the opponent's court, and to prevent the same effort by the opponent. A team is allowed up to three touches to return the ball across the net, and individual players may not touch the ball twice consecutively.

Beach volleyball is a team sport played by two teams of two players on a sand court divided by a net. As in indoor volleyball, the objective of the game is to send the ball over the net and to ground it on the opponent's side of the court, and to prevent the same effort by the opponent.

Sitting volleyball (sometimes known as paralympic volleyball) is a form of volleyball for athletes with a disability.