Domino

By counting all 28 tiles in a standard double-six set and comparing them to the 27 tiles in the image, the [6, 0] tile (blank and six) is the only one missing.

The excluded domino is number 0 and 1

By systematically cataloging all 27 visible dominoes in the image and comparing against the complete standard double-six set of 28 tiles, the domino [2, 4] is the only one missing from the set.

...

I inspected all 27 tiles and compared them to the full double-six set (all pairs 0–6). Every pair is present except the domino showing 4 and 6, so [4,6] is the missing tile.

By listing all pairs present in the 27 dominoes shown in the image, all combinations from [0,0] to [6,6] are accounted for except [0,6], which is absent. Each number’s occurrences confirm that 0 and 6 appear only 6 times each, while others appear 7 times.

By systematically identifying each of the 27 visible dominoes, the complete set present includes all tiles from [0,0] through [6,6] except for the [5,6] combination. The dominoes with 6 that are visible include [0,6], [1,6], [2,6], [3,6], [4,6], and [6,6], along with [5,5], but the [5,6] tile is absent from the arrangement.