Hi, I'm trying to book an overnight train from Prague to Krakow from the CD e-shop. However when I tried to select either a couchette or a sleeper, the following error message appeared, "Your request could not be granted. Train category and tariff details incompatible.".
I've two questions: (1) Why does the error message appear? If I just chose a seat reservation instead, it's fine. Does that mean I cannot book a sleeper? (2) Is it absolutely necessary for me to book online if I want the cheapest option there? Or can I just purchase the tickets at the Praha hln station when I arrive (I'll be arriving in Prague 2 days beforehand)? Thanks!