Domino

By systematically cataloging all 27 dominoes visible in the image, I identified each domino by counting the dots on each half. After checking all possible pairs from [0,0] through [6,6] (where the first number is ≤ the second number), the domino [2, 4] is missing from the set. All other 27 unique dominoes are present in the image.

The excluded domino is number 2 and 1

After listing and counting all 28 tiles in a standard double-six set and comparing them to the tiles present in the image, the domino [6, 1] is the only one not represented.

After identifying and listing the 27 visible tiles from the image and comparing them with the full double-six set, the only pair not present is 1 and 4; therefore the excluded domino is [1,4].

I identified all 27 dominoes in the image and compared them against the standard 28 dominoes in a double-six set. By checking which number pairs are present, I found that while dominoes [0,4], [2,4], [3,4], and [4,4] are present (showing 4 paired with 0, 2, 3, and 4), and dominoes [1,1], [1,2], [1,3], and [1,5] are present (showing 1 paired with 1, 2, 3, and 5), the domino [1,4] is not found in the image. The domino [1,4] is the only one missing from the complete set.