Just note that you need besides the Railpass (Eurail) a Reservation for Eurostar And iam sorry to say that but these Reservations are limited for Eurailpass user and are already sold out I recommend to find a alternative way to get from London to Brussels. 

Sorry a Eurailpass is for that route not a good offer :/ 


Maybe you cant pay such a amount with your selected Payment Method. 
e.G. The Finalprice is over your CreditCard Limit

Have you already booked your Eurostar reservations? Because there is no availability anymore on Eurostar from London to anywhere on Monday. You can check that here:

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

Not sure what is wrong with your payment. Contact customer service:

https://eurail.zendesk.com/hc/en-001/requests/new

Mention your start date, so they can prioritise your request correctly.

Also check with your bank whether there's a problem. Also try another credit card if you have one.

What more travels do you plan to make by train? As you will be travelling during the holidays season it is wise to check availability of seats on routes that require reservations before you buy a pass.

Eurail doesn't show actual availability before you have bought a pass. You must check that elsewhere. If you share your travel plans here then the Community will give advice where to check for reservations. 

Forget that €* for now this upcoming week. Plus loads of strikes announced in GB.

It MAY be the case that either the MAX sum to pay or the MAX nr of travellers is too high to process that in 1 go-I think have read/seen some reports on that some time ago.