Copy Database error Index was out of range. Must be non-negative and less than the size of the collection
"Trying to "Copy Database" from SQL Server 2016 to SQL Server 2012
  through SSMS."


You cannot copy a database from a 'higher' version (2016) of Sql Server to a 'lower' version of Sql Server (2012).  That's called 'downgrading' and it not supported.  There are numerous Internet posts on 'options' (like scripting individual objects, etc.) for doing this type of activity.

UPDATE 2/11/2017

There is an open connect item that appears to be addressing the same problem you're having