There's an issue on the Interrail website in these areas since today. Hopefully, they'll be able to resolve it soon.

Thanks for your help.  Much appreciated.  Unfortunately I can’t wait for them to resolve the issue as I am stuck somewhere I don’t want to be. 

If you mention the exact trains you'd like to book (departure, destination, date, time), then people can suggest alternative places to book.

I was trying to get to Paris from Lille but at the moment I am wondering if I should just go home if I can’t actually book trains etc.  

If you're currently in France, just go to a ticket office. That's also cheaper than using Interrail's service, which has an extra 2 EUR fee per seat.

Alternatively, you can book TGVs via Belgian railways (4 EUR fee per order) but you need a pass cover number, which is on the paper pass cover or if you have a mobile pass, you can request one here.

You can also call Belgian railways, which has no booking fee but phone costs instead; no pass cover number needed. Reservation is still delivered as a PDF via mail.

Super helpful (unlike app and website). Thanks.  

Just to update you, in case you will have to plan for more trips: the trips& travellers and reservation tool issue have been resolved as of yesterday afternoon. Cheers,