Have you completed the challenges "Register Blastoise from the Kanto Region" and "Register Kyurem from the Unova Region"?
I sent Blastoise from Red, Blue, Yellow, Crystal, Fire Red, Leaf Green, SoulSilver and Let's Go and didn't triggered it. And I have the challenges of sending Kyurem and Reshiram from Unova and Kyurem and Zekrom from Unova, but that other one didn't appear.