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1) reboot your iPhone, then close any app that automatically starts up after reboot
2) try the game connection with another Internet connection
3) check for updates to the game on the App store
When I have had this problem it usually is because of the connection. I find the game usually prefers using WiFi (instead of cellular service). Candy Crush can tend to be a bit picky about its connection and so I always check to make sure I get that green check mark in the bottom right corner of the screen when I first start to play - showing the game is satisfied that I have a good internet connection and sometimes I have had to switch to a secondary connection. You might also try turning off your cellular service (airplane mode) so your phone is only on WiFi and see if you can connect to Apple and successfully complete your purchase.
If this doesn't work - the last resort is to de-install the game and re-install it however if you do so you will lose all your saved boosters and lives. As long as you are playing the game thru your King.com or Facebook account you progress and gold bars will be saved.
Please let me know if this helps.