Message – “Error - Invalid Request Using Main ARB”

That typically means that you've rewritten (or deleted) the PCF record in ILF code. If you've rewritten the PCF record, be sure to re-read it with hold or Appx will get confused.

An 'arb' is an access request block - it's the internal representation of a file stream. When you OPEN a file, you get a new arb. Appx creates arbs for you when you BEG/END READ, or when you invoke a new process.