Fault 1062: 'Shipping method is not available'

[alokumar1]

• Version: 1.9.2.4
• Platform: ubuntu 18

was trying to set cart shipping method but it is giving this error

>>>client.cart_shipping.method(cart_id, {"method":"purchaseorder"})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/python-magento/magento/checkout.py", line 371, in method
    [quote_id, shipping_method, store_view])
  File "/home/python-magento/magento/api.py", line 202, in call
    return self.client.call(self.session, resource_path, arguments)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1112, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1452, in __request
    verbose=self.__verbose
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1154, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1170, in single_request
    return self.parse_response(resp)
  File "/usr/lib/python3.6/xmlrpc/client.py", line 1342, in parse_response
    return u.close()
  File "/usr/lib/python3.6/xmlrpc/client.py", line 656, in close
    raise Fault(**self._stack[0])
xmlrpc.client.Fault: <Fault 1062: 'Shipping method is not available'>

I have checked that this method is available by calling list method

>>>client.cart_shipping.list(cart_id)
[
    {
        "carrier": "purchaseorder",
        "carrier_title": "Purchase Order",
        "code": "purchaseorder_fixed",
        "method": "fixed",
        "method_description": None,
        "price": "0.0000",
        "error_message": None,
        "method_title": "Shipping",
        "carrierName": "Purchase Order"
    },
    {
        "carrier": "freeshipping",
        "carrier_title": "Shipping is pre-paid with UPS and added to your invoice",
        "code": "freeshipping_freeshipping",
        "method": "freeshipping",
        "method_description": None,
        "price": "0.0000",
        "error_message": None,
        "method_title": None,
        "carrierName": "Shipping is pre-paid with UPS and added to your invoice"
    }
]

Please help me I have searched many times but could not find anything, please help.