I'm not sure how you managed to get a "KLM marketed flight on KLM metal" when booking a Delta-issued ticket, nor how you were able to select a "Standard Plus" fare for the flight.

On the Delta website - which offers the flight under its DL codeshare and using a different "family" of fares - even when you pick the most expensive economy fare offered ("Refundable"), they want an extra C$187.51 to pick an Economy Comfort seat.

Delta simply does not offer the same suite of economy fare families ("Light", "Standard", "Standard Plus", "Flex") even for KLM flights, so I don't understand how, on a 006 ticket, you appear to have been offered a choice from the KLM suite of fares.

Notwithstanding that you booked this via an agency, I simply don't understand the mechanism by which you were able to select a KLM-branded "Standard Plus" fare. However, if that's what your agency allowed you to do, then you should take it up with that agency. If the agency offered you a fare including free Economy Comfort seat, then you must get the agency to provide assistance in securing said seat. I can see absolutely no evidence that Delta offers a fare that includes a free Comfort Plus seat, so you really must verify what you bought and/or what your agency promised.