If you can't find any train on the morning of the 8th of October, then there are no pass holder seats available anymore on those departures on that route. 

You can check if there ar seats available from Amsterdam and take the IC from Brussels to Amsterdam. In that case remember to have enough time for the Eurostar check-in.

You need a Pass Cover Number for making the reservation. You can generate your  PCN here:

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

Thank you. Why is that every time I tried to generate pass cover number through this link with the same mobile pass number it gives me different pass cover numbers. Which one should I use to book the seat reservation

It doesn't matter which number you use. But you should generate more numbers than you have mobile passes.

Thanks but what do you mean I should generate more numbers thank I have mobile passes ?

There are 5 of us with mobile passes 

As far as I know you only need 1 PCN to make the reservations even for several persons but if you have 5 passes you might get 5 PCN. You wrote that you used the same mobile pass to get several PCN and that's what confused me.

You can use local trains to Lille (no reservations) and then a TGV from there. Compulsory reservations at10 or 20 euros each depending on how full the train is.

Thalys I presume? Avoid it, it has very expensive and very limited interrail seat places.

As a Belgian, I advise you this:

Take a train to Kortrijk/Courtrai in stead. There’s a direct one that day, that leaves every hour at hh:27 in Brussels-Nord, hh:32 in Central and hh:39 in Brussel-Zuid/Midi. (Or you can go via Gent-Sint-Pieters with an additional interchange, it’s about the same time)

In Kortrijk you have a local train to Lille at hh:13. If you take the direct one Brussels-Kortrijk, it should be a well timed interchange. And In Lille you have plenty of ordinary TGV-trains with much cheaper reservations to Paris-Nord.

Normally you can book those TGV’s Lille-Paris via the same b-europe website.

Please note that there are two main stations in Lille: Lille-Flandres and Lille-Europe. But only a five minute walk away from each other.