When i select product in purchase order this error is showing:

File "/home/odoo-10/odoo/odoo-10.0/odoo/models.py", line 5525, in onchange
    record._onchange_eval(name, field_onchange[name], result)
  File "/home/odoo-10/odoo/odoo-10.0/odoo/models.py", line 5423, in _onchange_eval
    method_res = method(self)
  File "/home/odoo-10/odoo/odoo-10.0/addons/purchase/models/purchase.py", line 790, in onchange_product_id
    self._onchange_quantity()
  File "/home/odoo-10/odoo/odoo-10.0/addons/purchase_requisition/models/purchase_requisition.py", line 276, in _onchange_quantity
    res = super(PurchaseOrderLine, self)._onchange_quantity()
  File "/home/odoo-10/odoo/odoo-10.0/addons/purchase/models/purchase.py", line 831, in _onchange_quantity
    price_unit = self.env['account.tax']._fix_tax_included_price_company(seller.price, self.product_id.supplier_taxes_id, self.taxes_id, self.company_id) if seller else 0.0
AttributeError: 'account.tax' object has no attribute '_fix_tax_included_price_company'