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Thanks for the comment. Only the first domino located at the bottom of each circle is placed at the angle that tilts it to topple. So, when you click the Unity play button or evaluate WL UnityPlay[], the domino show will start. In the above code, it is the following.

"EulerAngles" -> {If[i == 1, 20.0`, 0.`],-(360/(2 Pi))*2 (i - 1) Pi/dominocnt, 0.`}