I should have reported that the command bound to the keyboard shortcut is "xfce4-session-logout --suspend". When run from the command line, I get a popup window with the message
"Received error while trying to log out GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs: Type of message, "(yb)", does not match expected type "(b")"" and the command line message: "Received error while trying to log out, error was GDBus.Error:org.xfce.SessionManager.Error.Failed: Failed to lock the screen." Celejar