If you are purchasing a course, some people find it easier to complete the 3D Secure authentication stage using a second device to ensure they do not lose their booking page.
If you are using a smart phone or tablet, you may need to open your app carousel to go back and locate your booking page.
If you are making a booking via the newsletter, you may need to reopen your email app to locate your booking page instead of your browser.
Unfortunately, if the booking is not completed fully, the booking will not be made and you will need to make your booking again.