If a frog is not attracted to another frog's croak, and never breed. What type of isolation is this?

The correct answer would be the behavioral isolation.

Behavioral isolation is a type of reproductive isolation in which mating rituals of one species are different from the other species. It includes mating dance, songs or voices made by males et cetera.

Males dance or produce sounds to attract females and they move to the next stage of the exhibition only when females also show some responses in her behavior.

In the given situation, the frogs may belong to two different species which do not know the mating rituals of the each other.