Per-Parity Patch Edit
The Robot Dance is a taunt used by MARS - ARK M.A.R.S.. You cannot purchase this emote alone, it is tied to the hero MARS.
Just like the weapons, you cannot use it on your regular agent even if you own MARS.
Post Parity Patch Edit
This currently not available for purchase in the Customization Menu. Originally, the taunt was obtainable by purchasing the hero MARS.