It looks like you may not have the permissions required to add a new extension. For example, limited users can see existing extensions, but don’t have the ability to add or change existing extensions. You can find your current user role by going to your profile and selecting the User Settings tab.
You’ll either need a standard user or admin to add a new extension for you, or an admin or the account owner to give you a higher user role/permissions.
If you are an admin or account owner, then you found a bug and should email email@example.com to let our team know
I hope this helps!