To proceed to payment you'll need to make sure that your web browser allow pop-ups and redirects from

Apple users in Safari: when using the web browser, please open your device’s Settings. Please select ‘Safari’ and then ‘Advanced’ and then ‘Experimental Features’. Please turn off the setting entitled “Block top-level redirects by third-party” 

Android users in Google Chrome: please check for a “Redirect blocked on this page” warning icon in your browser address bar. Please view the warning and click the link in the popup box to continue, or select “Allow” or “Unblock”.

To resolve this from an Android device please work through the following solution.