Win10 setup recognizes and doesn’t recognize a partition

I’m still running Win8.1. I created a Win10 setup ISO file years ago during the period when Microsoft was offering the upgrade for free. I now burned it to a DVD, rebooted, and tried to install Win10.
Disk zero refers to a drive that has no relevance to this discussion.
Win8.1 is on my C; drive (Disk 1 Partition 1). When setup asked me where to install Win10, I specified Disk 2 Partition 2, which is 500GB. Disk 2 is a basic GPT drive. That’s the part where setup recognized the partition.
I got an error message that setup was unable to create a new partition or locate an existing one. That’s the part where setup didn’t recognize the partition.