Where is the button to collect the reward from the event?
Tap on the Events building on the World Map, find the Colossus Awakening event and tap on it.
If you had minimum requirements to claim leaderboard rewards (5+ wins) and depending on your best score ranking, you should be able to collected them in that section.
Also, a day after an event ends, that screen can no longer be opened and all rewards go to your Inbox, so if you don’t claim then straight away don’t worry, they should appear in your Inbox.
Hope this helps!
Have a great day,