[Edit 13/03/2023]

I think I find a solution.

1) First, you have to create the Internal Transfer, and let it be "Draft".

2) Once inside, click on the OPERATION button, up-right of the form.
You're now in the model "stock.move" instead of "stock.picking".

3) Create an Excel file (xlsx) for you to import in that model Stock.Move with the fields :

- Product --> Full name of your product ([internal ref] Name).
Example for me : [ZL00.ASS.002] Cable - PCB LED

- Demand --> Quantity you want to move
Example for me : 29,00

- UoM --> Unit of Measure
Example for me : Unit(s)

- Description --> SAME information than "Product" field.

4) To have the right informations, I simply export the fields
PRODUCT  and  QUANTITY ON HAND
from the "product.template" model (Inventory --> Products button --> Products)

5) Be careful that if you want to change the quantity from the field "Demand", you have the line you want to change to get the unique ID of that line, and change the quantity there.
If you don't do that, you will simply add a line with the new quantity.

6) Once you have imported the Excel file, click on the "Mark as to do" button.

7) Click on "Check availability" button, and check that the "reserved" quantity match the "Demand" quantity. 

Hope that it will help.

To import an Excel file for an internal transfer in Odoo, you can follow these steps:

1. Go to the Inventory module and click on the Internal Transfers tab.

2. Click on the Import button and select the Excel file that you want to import.

3. In the Import window, map the columns in the Excel file to the corresponding fields in the internal transfer form. The fields that you need to map are:

• Product Code: This field should be mapped to the Product Code column in the Excel file.

• Qty: This field should be mapped to the Quantity column in the Excel file.

• Source location: This field should be mapped to the Source Location column in the Excel file.

• Destination location: This field should be mapped to the Destination Location column in the Excel file.

1. Once you have mapped all the fields, click on the Import button to import the data from the Excel file into the internal transfer form.

If you encounter any errors or issues during the import process, you can check the log messages for more information. You can also try exporting the internal transfer form to an Excel file and using that as a template for your import. This can help you ensure that the data in the Excel file is formatted correctly and matches the required fields in the internal transfer form.

This is what i usually do

1. Create an empty transfer and save as a draft.

2. export that transfer with import-compatible export selected (what you need here is the external id of that record and it is selected by default)

3. Populate a new excel file with these columns

• id (external id of the record you just exported)
• move_ids/product_id/id (external id of the product)
• move_ids/product_uom/id (external id of uom)
• move_ids/location_dest_id (destination location)
• move_ids/location_ids (source location)
• move_ids/product_uom_qty (quantity to be transferred)

4. Import

anything being imported here has to already exist in the database here except for qty. You can get the external ids of the product from product variant page not the template. hope you know vlookup:)