Z, S, W, O, T, K, Q, G,?,?

The correct pattern is + 6³, + 5³, + 4³, + 3³, .....

So, 521 is wrong and must be replaced by (498 + 3³) i.e. 525.

The correct pattern in the series is + 1110.

So, 5686 is wrong and must be replaced by (4566 + 1110) i.e. 5676.

The series may be divided into groups as shown:

b e d / f ? h / j ? l

Clearly in the first group, the second and third letters are respectively three and two steps ahead of the first letter.

A similar pattern would follow in the second and third groups.

The number of letters in the terms of the given series increases by one at each step.

The first letter of each term is two steps ahead of the last letter of the preceding term.

However, each term consists of consecutive letters in order.

The given sequence is a combination of two series :

I. Y,T, O and II. B, G, ?

I consists of 2nd, 7th and 12th letters from the end of the English alphabet, while

II consists of 2nd, 7th and 12th letters from the beginning of the English alphabet.

So, the missing letter in II is the 12th letter from the beginning of the English alphabet, which is L.