Most of the times this is the case when you do not have a stable internet connection. Please try and connect to a good working WLAN network or mobile network using for example 4G network. Should you still have problems at the checkout of the MyPostcard app please try to uninstall and then reinstall the app or see if there is an update available at your app store. 

If the problem still persists please contact us at support@mypostcard.com.