Domino

By cataloging all 27 tiles in the image against the standard set of 28 double-six dominoes, the double-six tile (6-6) is the only one not present.

The excluded domino is number 2 and 1

By systematically cataloging all 27 visible dominoes and comparing against the complete standard set of 28 dominoes (all pairs of 0-6), the domino [4, 5] is the only one missing from the set.

No double-six tile (6-6) is present among the 27 shown dominoes.

By analyzing the dominoes in the image, I identified all dominoes containing a 0: [0,0], [0,1], [0,2], [0,3], [0,4], and [0,5]. The only domino with a 0 that is missing from the set is [0,6]. Additionally, counting the occurrences of each number confirms this: the number 0 appears 7 times (should be 8 in a complete set), and the number 6 appears 7 times in the visible dominoes (missing one instance). All other numbers appear the expected 8 times. The domino [0,6] is the only one not present in the image.