If you receive this message your inventory is most likely full.
To solve this issue you either have to free up some inventory space or extend your backpack using Backpack Expanders. But before you can buy some Backpack Expanders you still need to make some space to be able to receive them.
To free up some space in your inventory you could delete unnecessary items, craft Scrap Weapons, Scrap Metal and Reclaimed Metal into Refined Metal or sell some Crates to the Steam Community Market.