Hi,

This error occurs because Odoo cannot find a route or rule to transfer products from your main warehouse to the new hospital location you created. Since you set up the hospital as a warehouse or internal location to hold stock before payment, Odoo treats it as an internal location and doesn’t know how to replenish it when confirming the Sales Order. To fix this, go to Inventory → Configuration → Locations and ensure the hospital location type is Internal Location. Then, under Inventory → Configuration → Routes, create or edit a route that defines how products move from your main warehouse to the hospital location. Add a pull rule with the source location as your main warehouse and the destination as the hospital location, then assign this route to the relevant products or customer. This setup ensures Odoo automatically creates an internal transfer when items are sent to the hospital, and you can later create a Sales Order once the hospital uses the items.

Hope it helps.

Yes, I tried the Routes as well. 

Made separate Route for this. Adding also this optional partner address. 

I just don't understand, why everything worked perfectly until I made that warehouse and location... I also checked if something has changed in the Customer fields, but there everything is the same as to our other Customers.