Your Inventory Warehouse is connected to many other parts of ShipStation. You must first unlink an Inventory Warehouse from its three connected elements before you can delete it.
This process requires the following four steps:
Remove all inventory stock from Products attached to the Warehouse
Unlink the Warehouse from its Ship From Location(s)
Delete all Locations from the Warehouse
Delete Inventory Warehouse.
If any of the first three steps listed above are left incomplete, you will receive the following error when you attempt to delete an Inventory Warehouse:
All products with any stock level tied to an Inventory Warehouse must be removed.
Remove inventory directly in the Inventory grid ...
... Or from within the Inventory tab in the individual product's Product Detail window:
Click the equals symbol (=) under the Actions column and set the total quantity to '0'.
If you make this change in the Product Details window, remember to click Save Changes.
Next, you'll need to make sure that this Warehouse Location is not linked as an Inventory Source for a specific Ship From Location.
Finally, any Warehouse Locations tied to the Inventory Warehouse must also be deleted.