Heya!

As long as you have gained enough points to get that reward, you should be able to click it and it should arrive in your mailbox. 

Such as this:

If it's not there, try restarting your game. There is also a possibility that you already collected it from your mailbox and it is already in your inventory. If the problem still persists, wait a little bit so a support agent can look at your problem. If you want to have a faster response, I would recommend submitting a ticket to the support team which can be found here: https://goo.gl/NWMB1Z

Hope this helps!