Dear all,

Help me...
Any suggestions, how can i solve this...

I'm having the same issue. I've began farming for the next tier wagon but the slots space still doesn't increase. Very frustrating. Love the new feature and hope this gets solved soon

It is referring to your character inventory, not wagon inventory. The game requires you have space in your character inventory before you start, because there are items that can be acquired that go to your character storage.