You need to put the .pak the .ucas and the .utoc in the same path in order to use IOStore. If you only have the .pak it means you didn’t create it with IOStore in first place so it won’t work.
I’ve passed this step but I can’t get the pak contents in C++, despite being able to list it with UnrealPak and replicating the same code.
ETA: I’ve looked with the debugger, it looks like UnrealPak calls LegacyListIoStoreContainer() from there…