The Spool SFTP driver requires some special authorizations for the Spool started task userid and creators of files to be transferred using this protocol.
Release: Output Management Spool for z/OS 14.0
Besides the requirements when RACF is used as the external security package, there is also these requirements that apply to any external security package:
"For the SFTP driver to work properly, the Spool started task userid and the file owner's userid using the SFTP driver require read access to the BPX.SERVER and BPX.DAEMON resource profiles in the FACILITY class."