Skype for business is not generating conference ID

Hi, you do not see a conferencing ID because you don't have any Audio Conferencing licenses assigned to your users. Audio Conferencing licenses provide PSTN dial-in capabilities for your meetings, which is the scenario that uses conference ID's. Audio Conferencing licenses are not included with the E3 plans that you have, and must be purchased as a separate add-on for the users that you wish to provide with this capability. Once you have assigned an Audio Conferencing add-on license to a user, any conferences / meetings scheduled by that user will have a PSTN conferencing number appended to their meeting invites, along with a conference ID. 

Audio Conferencing add-on used to be called PSTN dial-in conferencing add-on, which perhaps makes it a bit clearer for you. Microsoft complicated the understanding of this license by renaming it to Audio Conferencing add-on. Without this license Skype users have web conferencing capabilities whereby other users can join their meetings by using the meeting join URL (no conference ID required which is why you don't see it), but for someone to join by telephone (where the conference ID is required) the scheduler must have the Audio Conferencing add-on assigned to them.

Kind regards
Ben