An AJAX HTTP error occurred using commerce coupon percentage
This seems an error in Commerce Coupon Percentage module. Your problem comes from the EntityMetadataWrapperException: when processing the AJAx petition something goes wrong and an Exception is throw. This is fatal, so Apache ends the petition with the status 500.

I think this a bug in the module code, so you may go the issue queue for this module and create a new bug report with your problem. 

By the way, the Commerce Coupon Percentage is only comptabible with Commerce Coupon 1.x, check your are using a 1.x release.