If you have not already figured it out the community challenges were disabled yesterday.  This is the statement that one of the developers made on their forums:

“We’ve temporarily disabled Community Challenges until we can address the exploits mentioned above — namely the challenges that literally require someone to press X 99 times in order to gain an exorbitant amount of XP and Stubs. We built Community Challenges in the hopes that you guys would be able to create a diverse array of situations and challenges that would offer unique bite-size gameplay moments that may be more elusive in a normal 9 inning exhibition format. Unfortunately, though, users in the community have turned the mode into a way to farm incredibly large amounts of XP and Stubs.

 Long term, this is bad for all players — especially those who do not micro-transact with us, as marketplace prices have steadily inflated over time. Meaning your hard earned Stubs buy you less and less over time. Additionally, in terms of how DD has played out, these exploits have had an exponentially detrimental effect on team composition. We will be re-imaging Community Challenges and re-activating it in the near future to prevent any further impact(s) on DD.”

In summary, the MLB 15 The Show Community Challenges were never intended to be used to farm stubs and have had a negative impact on the marketplace.  The dev team is looking for a way to implement them back, but this may be the last time we ever see them.