This is one of the three missions, you need to check the time. If I'm not wrong it's say that you have to won 15 level before Wednesday 6pm. I think maybe after Wednesday (which is today) 6pm only you can receive the rewards. My time here only 12.52pm now. 😊
@ChioMunz gotcha. You have nothing to worry about then if you have completed the 15 levels required. The pop-up will stay there until the event finishes at 18:00 EDT (23:00 BST). You won't receive any rewards until the third mission has finished.
That is right @ChioMunz you don't get a message about completing the mission. It is the same with the second and third missions. The only message you will receive is after all three missions have been completed.
It is a very poorly designed feature in my opinion.