“You need to buy a seat reservation to board”

You can buy a seat reservation online via B-Europe but you need a paper pass cover number. If you have a mobile pass request it via DM on Social Media of Interrail. Or at Ticket Offices or Call Center of SNCB/NMBS, SNCF and maybe NS International?

For domestic French reservations, you can also call NS. Also no booking fees and they do accept VISA. Remember the country code (+31)

RES seat only is 10 (highly unlikely you get that at short notice) or 20€ extra to pay- a normal ticket will cost at least 50/60