Leor1c, two likely problems...
- The paths being referenced in the error indicate you're running a dev version of 7H, not the one Alyza linked to in the tutorial (unless she's changed it and I didn't catch it).
- After talking with a couple people, it seems as if Code 15 references an invalid drive. It could be that you had FF7DISC1.ISO mounted as E: when you ran the converter, but now it's completely missing, or at a different letter. Posting the contents of FF7_GC.LOG might help us figure out where the game expects to find it.