Solution or Workaround
When the PAL file is missing or corrupt in one or more coverages, the following error message is returned:
"Unable to append cover .pff file (APFILS)
Bailing out of APPEND..."
1. MAPJOIN groups of coverages to isolate the suspect coverages.
2. BUILD or CLEAN the coverages.
MAPJOIN should now process without the error message being returned.
It is also possible that the mask (MSK) file in the coverage directory may be corrupted. It can safely be deleted, and then run BUILD or CLEAN to update the topology.