Cannot add an account [non domain account ] to SQL Server 2012
It's probably because that's not the way that the SQL Server instance is seeing the application.  Attempt to use the application.  This should log an error in the SQL Server's ERRORLOG file when the application isn't able to connect.  This will tell you how the SQL Server is seeing the login.  Just copy the login from the ERRORLOG file and create a login for it.