In IT there are still the strictest measures anti-covid in all of EUR. Yes, sleeper coupes are only sold as whole set. The app from EUrail has many limitations.

It is so often said here that it is a sneeky workaorund to REServe via oebb.at=austrian rails,. but I dk if it also works for such sleepers.

Thank you for your reply.

I have since found other people on here discussing the same problem and apparently it is not a high priority to do anything about it. It appears it is not a Covid related problem and other sites are selling 2 berths in the same compartment but you can’t use your pass on these sites.

I think a notice should be put on the page about the cost of the night train as the way it is done makes it more expensive than just buying the ordinary tickets for the 2 person berth. It seems strange that they are making travel more expensive with the pass than without it! Also EURAIL are halving the train’s capacity with the compartments they sell which isn’t great environmentally.

Also frustrating that instead of using one travel day I will have to use 2 taking the day trains. 
Rant over. Hopefully in time it will be sorted out.

You could book your sleeper by phone via Belgian railways.