Check the Plugin Firewall > Plugins Script|File Whitelist Text Area > do you see this whitelist rule > /indeed-membership-pro/public/paypal_payment.php? If not, then add it and test the discount coupon. Note: Plugin Firewall whitelist rules use this format: “each plugin script/file path that you add MUST be separated by a comma and a space. Example: /plugin-folder-name/example-file-name.js, /example-plugin-folder-name/api/paypal-ipn-script.php, /another-example-plugin-folder-name/example-script-name.php.”
A plugin skip/bypass rule may be needed, but let me know if the Plugin Firewall whitelist rule does the trick or not.