I think they meant the "b-x, z" part.
It means the available letters are all the letters from b to x, and z. A and y aren't available.
_ a _ _ _ _ / t _ _ / _ a t
AVAILABLE LETTERS: b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, u, v, w, x, z
PHRASE: CARLY SHA
NO: Y