Help with a problem

The game I play is not a solo mode, my game character has already reached the other world, but my NPC Ava has no currency exchange

Unsure what you mean. This sounds like a POE2 question?

The currency exchange NPC in POE1 is automatically available in town. I'm pretty sure he does not need any progress to unlock the currency exchange and it's only missing on SSF modes. It could be that a few upgrades are needed, I don't remember exactly. If it's not instantly available, it is certainly very early in the town progression.

In POE 2 Early Access you need to go through the first three acts of the campaign twice: Once on standard and the other on cruel.

If memory serves, the currency exchange becomes available in Act 3 normal, but once you go through the portal and start the second round on cruel, the currency exchange becomes unavailable until you're back in Act 3 again (on cruel).

I'm not sure if this is intended or just an early access bug. In any event, you will regain access to it again when you are done with the campaign.

Important
If you've gone through all 6 acts (3 normal, 3 cruel) and you are in the endgame, you can also fast travel to your hideout from a waypoint and access the currency exchange there.

Hope that helps! GLHF :)