If you cant offload any cargo that you sold or any mission cargo at its destination. Then this guide is for you!
First you need to assign more storage zones, other wise the crates pile up at the door and once its full, everything stops. Use the tool and select the zones and see that here are many categories. Make some for mission cargo and some for you water, food etc items.
If you order more than your ship can hold, that might be a problem. I only know for sure is the game checks for cabin space so it might check to see if you have cargo space before allowing you to order to much… not sure on that tho.
But if your cargo bay door is full of stuff, you need to make more zones.