Hi,
You cannot use a SQL Server 2016 product key for SQL Server 2008 R2, you would need a SQL Server 2008 R2 product key to activate it.
Also note that SQL Server 2008 & SQL Server 2008 R2 are no longer supported.
If you have any further questions related to SQL Server, you may also ask the experts in the dedicated SQL Server forums over here:
https://social.technet.microsoft.com/Forums/sqlserver/en-us/home?category=sqlserver
(Please don't forget to accept helpful replies as answer)
Best regards,
Leon