1. Use transaction code: KA02: enter G/L account(Cost element) and in default acct assinmt TAB,assign the Cost Center and save.

2.Check the field status of the account in FS00. If this account requires CO object as mandatory then check whether a cost element is created or not , this error normally occurs when CO object is mandatory but relevant cost element is not created.Example: CO object may be cost center, cost element, WBS element

3.Use t.code OKB9 and assign G/L account to cost object with ur company code and save.

For more check the link

http://www.sap-img.com/financial/requires-an-assignment-to-a-co-object.htm