Hi James,
If it is just a warning in the prerequisite checker, you can ignore it as mentioned in SAP Note #1486772 - SAP Systems on Windows Server 2008 R2:
------------------
When you install or copy your SAP system on Windows Server 2008
R2, and the SAPinst Prerequisite Checker shows a warning that
your operating system version is not supported, ignore this
warning.
------------------
If it is an error, then in which phase is this happening? It might be due to old kernel version or old installer version. Are you using SWPM to perform the installation?
Best Regards,
Daniele