If you are unable to start a new challenge, first make sure you don't have an active challenge.
1. Navigate to your GPI.
2. Under your GPI graph will be two sections "Active Challenges" and "Recommended Challenges" (There might be a banner covering the challenges that you'll need to dismiss)
3. If you already have an active challenge, you can choose to cancel that challenge and start a new one.
If you don't have any active challenges,
1. Sign out of the application and the website.
2. Sign back into the application and website.