WordFence Learning Mode Setting
If you have the WordFence Security plugin installed and are experiencing failed payments with messages similar to “Unable to complete request. Please try again.“, this can be attributed to a WordFence setting. You will need to put WordFence’s Firewall settings in “Learning Mode”, clear all caches and minifications and attempt the payment again.
Putting WordFence in learning mode, then adding the requests it displays (similar to the screenshot below) to the allow list, should fix any issues associated with WordFence.