There are 9 png file in the zip, all of them are checkmate excersises You have to solve them, where it is the white turn, and finish the game in one step. These are the final steps in order:

QH7QA8KG7PD5BB5KB6RA8RH8QH8

This is the cypherrtext:

F^mY;L?t24Zk.m^-hnWl,[l)[ku

Both of them are 27 characters, but the final solution is:
- divide the ciphertext by 3 characters into 9 parts,
- take the numbers from the check solutions,
- rotate the divided part in order with the check solution numbers with ROT47 (in cyberchef),
- get the flag from the 9 part.

7
8
7
5
5
6
8
8
8

F^m
Y;L
?t2
4Zk
.m^
-hn
Wl,
[l)
[ku