It's possible there's something a bit off
For example, I had this issue last year when I was remaking Kiki's preset. I changed all the files from .jpg to .png while I was doing this, and I didn't modify the eye file. Well, in the preset maker the eyes still showed up (even though the code had the eyes set to .png while the eye file itself was still .jpg). When I was finally done and tried to export, I got the same error message you did. So in preset maker mess around a bit between preset/custom etc and see if anything stops showing up (then you'll know what to fix)
Or you can try just switching it to another folder