The text “Purchase Failed” will appear
steam overlay enabled? credit card limit reached maybe?
what mean “steam overlay enabled”
lol, really? Then you can rest assured that AGS will soon undergo emergency maintenance on such matters. 
Steam overlay must be enabled in your game settings on steam. It should be on by default but if it got turned off some how you would receive the “Purchase failed” error.
Steam Support :: Steam Community Overlay.
Additionally if your account has not received trusted status yet you will not be able to buy the larger crystal packages due to those restrictions.