For some reason, it says thta the inventory is full. I uploaded 600 items to it (are in inventory), it only shows 400 on dispenser plus it says that's the limit. Please help

Because you are only limited to 300 maximum items.

Every 100 inventory space from 300 and up is unlocked with $0.4.

If you want extra space for tf2/dota2 inventory, you need to provide the backpack expanders.