Server 2012 R2 Essentials Activation

I'm trying to reinstall and activate Server Essentials 2012 R2 using an OEM key. Everything keeps saying to install the trial and then activate with my key using slmgr.vbs. Everything points back to this article: https://blogs.technet.microsoft.com/blainbar/2013/03/06/article-converting-evaluation-versions-of-windows-server-2012-to-full-licensed-versions-with-valid-product-keys/

And specifically the line that reads: For Windows Server 2012 Essentials: You can convert to the full retail version by entering a retail, volume license, or OEM key in the command slmgr.vbs.

If I run: slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx

It errors out with this: 0xC004E016 The software licensing service reported that the product key is invalid.

I've found this over and over in forums but never a solution. Can anyone help with this?