This issue is most likely caused by setting the Maximum Transaction Limit for the Mobile Donation form to $0.  To correct the issue: 

  1. Go to Communities > Special Events > Friends Asking Friends > Kintera Thon
  2. Find your event
  3. Hover over the event name and select Website Info
  4. Click Fee Structure
  5. Scroll down to the section labeled: Maximum Transaction Limit
  6. Delete the entry for the Mobile Donation Form
  7. Go back to the Event Website Creation Checklist
  8. Click the green Publish This Event Now button 
  9. Refresh the site and you should now be able to submit the form

If the solution provided did not resolve the issue, Chat With Support and reference this article.