Can't Create a Skype Meeting In Outlook 365 Calendar

Thanks for the clarification, so the 'Add Skype meeting' button doesn't appear, when in the calendar and clicking New:   

I'd check this is consistent across different browsers and double check the licence is fully assigned, which sounds like it is. 

These are the things Microsoft say what to check if an online/skype meeting option isn't available:

• You’re not using a https://products.office.com/en-us/office-system-requirements of Outlook Web App.

• Your Office 365 subscription does not include Skype for Business.

• You’re logged in as a user who hasn’t been granted a license for Skype for Business.

Also, you could try this, as mentioned https://answers.microsoft.com/en-us/msoffice/forum/msoffice_sfb/office-365-add-skype-meeting-option-missing/e9187c74-1651-48c4-97a8-1652b7b9406a?auth=1, depending if this is only affecting a particular user (or is it not working for anyone):

1.  Remove the Skype for Business license from the affected user.
2. Wait for about 2 hours.
3. Assign the license back.