Cancellation/ exchange return ticket / outbound

Hello eveyone !

I struggle to know  whether or not i can exchange (cancel if needed) my return ticket / outbound (Strasbourg - paris). 

On your website is written : "all seat reservations are subject to the refund policy of the individual railway company – you can review this before confirming your cancellation. Booking fees are non-refundable."

I don't get if I can get reimbursed (i paid 20€) if i change my return ticket (location and date), and if yes, how ?

SNCF told me : because you booked on another platform (interrail) we cannot see anything about your reservation, please reach Interrail team.

Could you please enlighten me ?

Thanks a lot !

Sophie .