WP Simple Pay uses WordPress’ REST API to process payment forms and the REST API endpoints must be accessible for payments to be processed.
Disabled REST API
Some WordPress security plugins disable the WordPress REST API by default. Ensure the WordPress REST API is accessible so WP Simple Pay can process payments.
You should be able to access
https://your-website.com/wp-json/ if the REST API is available.
Invalid jQuery Script
Ensure the one and only version of jQuery being loaded by your website is the version that is included with WordPress.
ModSecurity can also block internal API requests. See our documentation article on ModSecurity for more information.