Try making the reservation at Interrail.eu or, to save some money on the reservation fee, which is 2 EUR per person and train at Interrail/Eurail, make the reservation through b-europe instead, see info below.

The reservation fee at b-europe is 4 EUR per booking, no matter for how many persons or trains.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

OOI if you have a UK credit card does that mean you are a UK resident? If so you should be using an Interrail pass and an Interrail account. Eurail is for non-European residents so it is no surprise a UK credit card is rejected, and you need to use the Interrail side of the service.

If  a UK resident  inadvertently sources a Eurail pass they will need to exchange it for an Interrail pass at a cost of 15 euro. More info on exchanges available on the Eurail and Interrail sites.