Hi all! I am trying to book the sleeper from Bucharest to Halkali. I see that this is possible to do over the Romanian railway company website, but trying to do it from the interrail.eu website produces a message saying there are likely no tickets left for pass holders. This error message appears regardless of the date I select though, so is it even possible to book via the interrail website? 
 

i just ask because I don’t want to pay more if it is possible to use the pass!

thanks!