Had an issue recently where Microsoft Teams users despite having Audio Conferencing enabled on their user in Office 365
The Teams and Skype Admin centre was displaying Audio Conferencing “Off” for the user.
When we went into the user and tried to Edit the Audio Conferencing settings we get an error
“We weren’t able to enable the conferencing bridge”
Fix
The fix for this was to assign the user a “Skype for Business” licence as there seems to be a requirement for an SfB Licence for a Teams user to use Audio Conferencing.
Once we added Skype for Business Audio Conferencing was successfully enabled for the user 🙂
I think this will be changed in the future but for the moment if you come across this problem this should resolve it.
Martin Boam's Microsoft Blog 
I also ran into this problem, and applying the Skype for Business license fixed it for me as well.