Hi Hubeertoo,
By looking at the file attachments for the forums top JetAppointment searches and cross referencing it with some update logs I would have to say that the JetEngine is not included as one of the plugins and you will need to download that separately. Luckily if you do a search in the forum for JetEngine 2.11.9 you should be able to use this plugin to resolve your conflict. Please refer to crocoblock documentation to avoid any future problems.
Regards,
Jack