Like you enable email for an existing group in Exchange 2007 and Exchange 2010 from the Exchange Management Console, there is no way to create from Exchange 2013. On Exchange 2013 Admin Center you can only create a new Distribution group. In-order to enable email for an existing security or distribution group you can make use of the Exchange Management Shell.
But to enable email on a group you need to make sure that it is a Universal Group. using below one line command you can make any group mail-enabled.
Enable-DistributionGroup -Identity “DG-Finance” -Alias “DG-Finance” -DisplayName “Finance Group”