This is not a post-SQL function, this has to be issued as a post-session command or in a standalone command task.
Post-SQL can be used for "normal" relational sources and targets, not for mainframe files or databases.
DTLREXE is an executable program, rather than being SQL.
Chapter 6 of the PWX Utilities Guide covers DTLREXE.
The syntax you used looks OK.
Depending on how SECURITY is configured in the mainframe PWX Listener's .RUNLIB(DBMOVER), you might also have to specify a uid and pwd (or you might not).
Thank You Dan & Nico.