1. do you really have PAPer passes? That nr is on the backside of trip report, right hand high side: I + 8 digits. If it is not right, you have been sold extremely old stock
2. more likely is you have a mobile pass-these do not have this nr and has to be requested via special form at service.
3. This nr is mostly wanted to reserve via b-rail: you can also try via fone with sncf-they do not ask for it. AND find overveiw of seewulf with all advice on howtodo it out of app with the railways.

It really is a paper pass!

Did you buy the pass from Interrail or elsewhere?

Here you can read about where to find the Pass Cover Number:

https://www.interrail.eu/en/support/reservations/where-can-i-find-my-paper-pass-cover-number#:~:text=You'll%20find%20your%20Pass,and%20above%20the%20control%20area.

Thanks AnnaB.  We bought the passes from Eurail. I have found the numbers - they are just old/incorrect.

What route are you trying to make a reservation for? Almost all routes can be reserved without the need of Pass Cover Number. 

It’s Eurostar. 
 

Ok. What route and day?

Think I just need a new number. But Brussels or Amsterdam on 27th

You are writing that you have Eurail passes, but if you are from the UK you should have Interrail passes. 

You can call b-europe and make the reservation. No need of Pass Cover Number.

The phone call is rather expensive.

https://www.b-europe.com/EN/Contact/Contact-Centre

You might be able to call SNCF, but I'm not sure if they only book the Eurostar to Paris. You can try to call them first as it is cheaper than b-europe. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

Press #85 for English, no booking fees, reservations are sent via e-mail

To get your Pass Cover Number, if you still will need it, you need to contact Customer Support through this form and ask for the Pass Cover Number. Make it clear that they have sent you passes with incorrect pass cover numbers. 

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

Customer Support is currently overloaded with requests so you will probably have to be patient to get help. Please let Customer Support know what date your travel will start so that they can prioritise your request correctly.

There are no pass holder seats available on the Eurostar on the 27th of June, neither to Brussels nor to Amsterdam. There are seats to Brussels on the 26th and 28th.

To Amsterdam the first available seats are on the 5th of July.

Thanks Anna B. I’ve have been waiting for a response for over a week.  I’ve tried again, but now looking at alternative routes out of the uk that don’t need seat reservations. It’s just so disappointing and frustrating. 

Quite hard to find non reservation needed routes out of the UK as the Channeltunnel is the only rail connection.

Other options you may consider :)
Catch the Harwich to Hoek van Holland ferry (30% discount with a railpass)
Catch the NewCastle to Amsterdam ferry (no discount)