That error can usually be due to a previous Linux or some other OS being installed on the partition you are trying to install Windows 8.1 on.  The fix for this that works most of the time is to just delete that whole partition.  You will then see “Unallocated space”  just hit continue or install button without formatting, new or creating any partitions, and you should see the Windows 8.1 installer do it’s thing and finish up. windows 8.1 install error partition is gpt type