I think is not good to set tax field into readonly when you are in draft state. But you still want to do this you can change it directly in the sale_order.py file in sale module addons(line 687)

        'tax_id': fields.many2many('account.tax', 'sale_order_tax', 'order_line_id', 'tax_id', 'Taxes', readonly=True, states={'draft': [('readonly',True)]}),