According to the error there is a modification in your GTA SA that is not allowed. Restore your gta3.img to default and try again.
the error message says "Modified bobcat.dff in gta3.img not allowed"
so we have 2 options:
- Re install GTA SA to fix the gta3.img, and if it doesn't work then try the second method.
- Re download GTA SA from another source because maybe you have missing files in your current version.