Dimitry,
Adding to what Amlan has already said, Posting Area: R001 in particular is used to determine the 'Sales Revenue Account' for a business transaction. Hence, it is possible for you to have multiple revenue accounts (derived via sub-transaction) for one main-transaction.
In addition to the revenue account, you can maintain other assignments here as well, such as:
- tax codes
- business area
- CO assignment objects (cost centers, profit center etc. )
To resolve your error message, you would have to maintain an account assignment for the <ADID/CCode/Division/Main-tran/Sub-tran> combination as stated in the error detail.
thanks,
Usman