This is really frusturating. I am having the same problem wile i'm trying to transfer products from one warehouse to another. Even though they have the same parent warehouse (which shouldn't be necessary if you ask me) i can't seem to transfer them. Here is step by step of my problem, i would really appreciate if someone can help me;

- open Barcode app

- scan source warehouse barcode

- scan product

- scan destinated warehouse barcode

- error message: "This location is not child of the main location"

Here's the response I received from Odoo support, as I'm experiencing the same problem. It's only a stop-gap solution in my opinion.

Thank you for contacting Odoo Support. Please note that this is a known limitation of the Barcode app in version 13. By default, what is shown first in your list of warehouses (accessible by entering the Inventory app, clicking Configuration from the menu items, and selecting Warehouses from the drop down options) will show when you open an Inventory Adjustment through the Barcode app. You can change the order of the Warehouses by using the arrows to the left of the warehouse name (highlighted in the attached screenshot). Your other option is to manually create an inventory adjustment for a warehouse/location other than the top one from the list then using the Barcode button from the top right corner to take advantage of the barcode option.