Almost certainly it was saved without PDF compatibility. Modern .ai files are PDFs internally, which is how PatchForge reads them; a legacy or EPS-flavoured .ai is PostScript and cannot be opened. Re-save it from Illustrator with Create PDF Compatible File ticked, or export it as a PDF or SVG. PatchForge tells you specifically when this is the problem instead of silently doing nothing.
Sep 12, 2026