My sister can’t complete the challenge. She’s getting the “There are no tasks right now. Come back later for a new challenge.” Window pops up when she clicks the task window. She has one more quest to finish. I’ve searched Google but nothing there. She plays on an IPad, has turned it off and restarted the game but it hasn’t helped. Any ideas? Thank you. 

You only have a total of 30 days to complete the CC. She's getting that message because it ended a few days ago. I can't recall exactly which day - Tuesday or Wednesday earlier this week.

Thank you. I didn’t know that 

As LvlSlgr has already mentioned, the Challenge was concluded when the new day rolled Tuesday, May 23. Players only have 30 days to complete all tasks. They all must be completed in sequence and only when each are unlocked. 

So sorry your sister missed the deadline.