So, slight development: as I was screenshotting the error, I realized that this error was resulting from misunderstanding how the directory worked. I changed the path so that exports now target an "exports" folder in the x64 folder.
Making this change resulted in exports without error! So, it was a silly error on my part, not realizing the directory was looking where the aseprite file was saved. Oops.
As a slight aside, are there any plans to support indexed color mode?