Hi @milli4697, i checked your site and found out no issues with the course purchase please check :
http://screencast-o-matic.com/watch/cDnehuhLPm
but i can see there are some of the courses(
http://fibu.academy/?post_type=course&p=1242) which are in drafts ,there may be problem with these course as they will not show to the user in the front end and students will not be able to subscribe to these courses .
Please make sure that all of the courses are published and also tell your students to clear their browser caches before they access your site .