Before diving into the fixes, it's essential to understand the role of the product key in Windows Server 2008 R2 Standard. The product key is a 25-character code used to activate the operating system. It's usually provided by the manufacturer or retailer when you purchase a copy of Windows Server 2008 R2 Standard.
Try standard online activation:
By using the slmgr and DISM tools, you can bypass most GUI-related errors and restore your Windows Server 2008 R2 Standard environment to a fully licensed state. product key windows server 2008 r2 standard fix
slmgr.vbs /cpky
$ProductKey