You get this error message (Account XXXXXXXX requires an assignment to a CO object) when GL account is relevant for Cost posting ie Primary Cost element is created for the account. In this case, System is going to need a Cost Object (Cost Center, Internal order, wbs element etc) in the posting as a posting needs to be automatically done in Controlling.
What if this is an Automatic posting?
In case of auto posting (such as gain/loss posting), you will not be able to assign Cost object manually. In that case you will need to assign a default cost object to the account (OKB9).
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what i do to remove cost element assignment?