Hi,

odoo verify first in the product if income, expense accounts (and for stock input, output too) are defined, if not, verify in the category, if not in the 2 you have your error message.

And no, category doen't inherit of parent category, see the code :

def _choose_account_from_po_line(self, cr, uid, po_line, context=None):

     fiscal_obj = self.pool.get('account.fiscal.position')

     property_obj = self.pool.get('ir.property')

     if po_line.product_id:

         acc_id = po_line.product_id.property_account_expense.id

         if not acc_id:

             acc_id = po_line.product_id.categ_id.property_account_expense_categ.id

         if not acc_id:

             raise osv.except_osv(_('Error!'), _('Define an expense account for this product: "%s" (id:%d).') % (po_line.product_id.name, po_line.product_id.id,))

bye