Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
I used Mega-Rayquaza on Pokemon Showdown in the Ubers format, and I couldn't mega evolve Rayquaza. It works fine on the Anything Goes format, but not on Ubers. What is the reason for this?