Marcel, Lectora 17 now appears to no longer use frames/framesets so if you’ve LMS published your title and an embedded YouTube video is not showing an active full screen button then the LMS may be placing your title inside a frame. Talk to the LMS vendor and see if there’s a way to add the allowfullscreen attribute to their frame.
Marcel, I’ve already said what I did to get it to work i.e. it works for me. Just saying it doesn’t work for you provides no information that can be used to help you. You haven’t said what LMS you’re using, what version of Lectora you’re using, whether your LMS opens courses in a new window, whether you actually tried what I suggested…
here’s a sample course with an embedded Youtube video built with Lectora 17.0.4.
The fullscreen button works when published to Scorm 1.2 in moodle 3.1 (new window), although moodle displays the course in its own frame. If it’s not working for you, you should talk to your LMS Administrator.