Domino

The excluded domino is number 2 and 1

By systematically identifying all visible dominoes in the image and comparing against the complete standard set of 28 dominoes, the domino [0, 1] (or equivalently [1, 0]) is not present among the 27 shown dominoes. All other combinations of numbers 0-6 appear to be accounted for.

After cataloging all 27 dominoes visible in the image and comparing them against a standard double-six set, the [4, 4] tile is the only one missing.

After counting the visible halves, the counts for the numbers show that 2 and 5 each occur one time fewer than expected in a full double-six set, so the missing domino is [2,5].

After identifying all 27 dominoes present in the image, the complete set of dominoes containing 0 is: [0,0], [0,1], [0,2], [0,3], [0,4], [0,5]. The only missing domino with a 0 is [0,6]. Additionally, among the dominoes containing 6, the following are present: [1,6], [2,6], [3,6], [4,6], [5,5] (which confirms [5,6] is not the missing one via other deductions), leaving [0,6] as the only gap in the standard double-six set.