Challenges are competitions that take place during a specific time period. There will be times when there are no challenges scheduled. But when a Challenge is scheduled, it will appear in the app as "Available." Typically, a Challenge becomes Available in the app about a week before it is set to begin.
Remember that your Actions won't count towards the Challenge until the Challenge actually starts and you must be a Member of the Challenge. Check the Info section of the Challenge to confirm these details.