Hello Big Choi,
I already checked that before. No, the cost center is not locked for posting with Revenue.
When the revenue account has the cost element category as 11 (revenues), the system throws error that the "Account 4XXXXX requires assignment to CO object". But if the cost element category is 1, then there is no error and the document parks. I am trying to understand why is the system generating the error with cost element category 11.