UPDATE:
I found a workaround but I have no idea what was happening. When I tried an implementation, I got a slew of errors about not enough locations remaining and stuff I fundamentally don't understand.
I moved some of my ports from one of the Arty Z7's chipkit connectors to one of its Pmod connectors. That allowed me to create a bitstream successfully.
(But I don't like not knowing what was wrong
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
)