Why are we unable to post order, when system shows message ‘Posting Unsuccessful’?

Suggestion: System shows the error’ Posting Unsuccessful’or ‘Rate / clg. rate not found.’ If the Sales order items are not defined in pricelist. Users should ensure that the Sales order items are defined in the pricelist before posting the orders. Some times the error’ Posting Unsuccessful’ is due to the Pending orders not found for the specified filter parameter, order(s) either not confirmed or already been posted. Posting log*