Some people can’t complete hard challenges.
But I got a cheat for you of how to complete hard or easy challenges.
Step 1: look at the level or score you need to get to
Step 2: play some easy game and reach the level, score or what you have completed.
Step 3: Click on the highest level (I have chosen 30) and play it for few seconds. (If that doesn’t work then play that level till it completed)
Step 4: go back to beat street (or any place to do a challenge) then click on the challenge.
Step 5: Go to the game then it will say you completed your challenge while it was still loading!
Then collect your reward!
(To do another challenge, you will have to repeat this again)