Troubleshooting

"More than one file is matched to the same post" Two or more staged files are proposed for the same destination, so running them would make each overwrite the last — and only the final one would survive. The run is blocked until you resolve it.
"This file is also used by a post outside this batch" The file you are about to overwrite is also what another post points at, and that post is not in this batch — so the replacement is refused rather than silently changing a page you did not select.