Best always mention the dates of which you are trying to find the connection.

The Interrail planner is flawed and often wrong, since it doesn’t get updates a lot.

Anyway, best use the planner from Deutsche Bahn, it has correct information for most of Europe is in it. or the local railways planner.

For your specific request, I don’t think there are trains from Athens to Sofia this year. But I’m not well travelled there, someone else might know better. Maybe in Summer there will be a connectoin.

Most other people recommend the bus, although not included in your pass.

Thank you for you're quick answer. So this isn't possible to do in one day? Does it has to be a direct connection? Or can I have several stops? And when I do that, do I pay more or something in Addition? At the moment I’m trying to find a connection on the 21st of February.

Maybe perhaps but NOT all the way by train. There is since a yr or 2 a GAP (that means no train over the rusty rails) from last stop in GR till border point=Kulata where BDZ=Bulgar trains run again. 

Best site for info on all this: seat61.com It is very much pro-train so will suggest very cumbersome and time eating train-bus (taxi/walk)-train combo”that lengthen trip time enormously and will perhaps even cost more.

People with a little more sense simply book a BUS from Thessaloniki to Sofia. Around 20€-same has been asked here dozens of times.

To prevent next great idea: the old classic route from GR to DE etc even had through trains once-but that is long gone and over the borders of all the parts of what was Yugoslavija are also NO trains and often not even e buslink.The common route from GR to mid-EUR is by ferry to IT and then TrenItalia.

You may even want to reconsider that pass-or a shorter one, as normal fares untill you reach the ROM-HU trains are quite low. Plus that trains, as not being upgraded for many yrs, tend to be old, rusty, vandalised etc.