Hi there,
It sounds like you installed the free version over the pro version. Please reinstall the pro version again (without uninstalling anything). Your data will not be lost in the process.
Also, to avoid this happening again in the future, go to Extensions >> Manage >> Update Sites, search for FAQ Book and delete the Update Site for the free version of FAQ Book.
To know which one to delete, look at the link below the title. The link for the free version must be
http://update.minitek.gr/joomla-extensions/minitek_faqbook.xml
while the link for the pro version should be
http://update.minitek.gr/joomla-extensions/minitek_faqbookpro.xml