Attempts to install Windows 8.1 onto an Early 2011 MBP are failing
Ignore the message. You do want to select Custom: Install Windows only (advanced). See image below.



The answer you are referencing was posted Jun 11 2015. This answer is basically the same as the earlier answer I posed on Apr 9 2015, except for a few omitted steps. The procedure you used might work provided a BIOS booting operating system was previous installed on your Mac. The answers I posted here and here were tested for Windows 7 and/or Windows 10, but not for Windows 8 or Windows 8.1. You might consider trying one of my posted answers. In any case, the Window Support Software for your Mac should be downloaded by using the Boot Camp Assistant installed on your Mac or directly from the website Boot Camp Support Software 5.1.5621. This software should be used with any 64 bit of version Windows you install (including Windows 10).


  BTW: You and download and try Windows 10 without having a Product Key. Use Safari to download the Windows 10 ISO from this Microsoft website. You can buy a license after installing and testing Windows 10.