To mask the "Mark as Done" button in a manufacturing order if there is a backorder not done in transfer, you will need to create a new computed field in the manufacturing order model that checks for the presence of a backorder not done in transfer and then use this field to control the visibility of the button.

Here's an example of how you can do this in Odoo 15:

1. Create a new computed field in the manufacturing order model to check for the presence of a backorder not done in transfer. For example:

   has_backorder = fields.Boolean(compute='_compute_has_backorder')

   def _compute_has_backorder(self):
   for order in self:
   order.has_backorder = any(move.state != 'done' for move in order.picking_ids.mapped('move_lines').filtered(lambda m: m.product_id.type=='product' and m.location_dest_id.usage == 'internal' and m.location_id.usage != 'internal'))




2. In the XML view of the form, add a new attribute to the "Mark as Done" button called "attrs" and use the "has_backorder" field to control its visibility
   
   
   This way, the button will be invisible when the "has_backorder" computed field is set to True, indicating that there is a backorder not done in transfer.