Intercity.pl is funky - which is expected. Better information on bahn.de
It is train EN 407. Note that to designate Prague, you may have to use "Praha hl.n." or something similar to specify the main train station in Prague.
There is a notation in the bahn.de details for the train: not every day 6. May until 2. Jun 2024. However, not able to make much more progress toward buying a ticket here.
Best results were on the Czech railways website: https://www.cd.cz/en/default.htm
However, it appears that it is not a night train through to Prague - there is a change from a sleeper to a conventional train at Bohumin with a long layover. The trains are EN 407 Chopin from Krakow, then IC 518 Ostravan from Bohumin. Price for a 2-person sleeper is 1870 CZK.
It is a challenge to configure the journey correctly even on the Czech rail site. So who knows whether what I pulled up is correct - but that seemed to be the only itinerary that fit the bill. Note that EN 407 Chopin is actually a night train from Warsaw to Munich.
You might try inquiring directly to Czech rail - the contact form is at: https://www.cd.cz/en/kontaktni-formular/
You also might try posting a query here on the Czech Republic forum - if Jason of LivingPrague.com sees it, he may be able to provide more info. Or try to contact him directly.
Here is the relevant page for the sleeper from the Man in Seat 61 - his info is that the sleeper does run Krakow to Prague: https://www.seat61.com/trains-and-routes/prague-to-krakow-by-train.htm Perhaps his info is out of date?
But he refers to PolRail.com, and they show the train - you might try contacting them: https://booking.polrail.com/index.php?main_page=page_5&from=Krakow&to=Prague&date=30-05-2024&language=en
So more to investigate. If the change from sleeper to regular train in the middle of the night is correct, then the best option might be the afternoon, train Krakow to Praha, which only takes approx. 6 hours.
Edited to add: It occurred to me to try to buy a sleeper compartment Krakow - Munich [MUENCHEN OS] on EN 407 and those do come up on intercity.pl - so additional indication that the sleeper is currently not running to Prague.