Domino

The excluded domino is number 2 and 2

By systematically cataloging all 27 visible dominoes in the image and checking against the standard 28-piece double-six set, the domino [2, 4] is the only one missing. All other combinations of numbers 0-6 are present in the image.

After cataloging all 27 dominoes in the image, the only missing tile from the standard 28-tile double-six set is the [6,6] double.

Counting the pip faces in the image shows the 6-pip appears two fewer times than it should (6 times instead of 8), so the missing tile is the double six [6,6].

After systematically identifying dominoes in the set, the domino [0, 6] is missing. The set includes dominoes [0,0], [0,1], [0,2], [0,3], [0,4], and [0,5] (vertical domino in row 2, far right), but the tile with blank/zero on one half and six dots (arranged in a 2×3 grid pattern) on the other half is not present among the 27 visible tiles. All other dominoes in the standard double-six set can be accounted for in the image, leaving [0,6] as the single excluded domino.