Rear Curtain Sync cannot be activated on older model Sony cameras when also using the WL mode for off-camera flash.  This is because two functions located in the same Sony camera sub-menu cannot be activated at the same time.
Note.  Sony updated the flash menu to separate the WL feature from other flash functions on newer cameras such as the A7III and A7RIII.  On these cameras it is possible to activate the Rear Curtain Sync mode when using off-camera flash with the WL mode activated.