To breakdown the message for you:
"The General Posting Setup does not exist" = General Posting Setup controls how the numbers flow to the G/L. You do not want to post a transaction that is not fully setup. This is where you can see what combinations (Gen. Bus./Prod. Posting Groups) are valid on your transactions.
Values:
Gen. Bus. Posting Group='Domestic' = This value comes from the Customer or Value on the Sale/Purchase Document
Gen. Prod. Posting Group='' = The '' means it is blank. This value comes from the Item or G/L account used on the document's lines, it is currently missing from this transaction.
Resolution:
Whatever Item or G/L you are using is not fully setup, so it cannot process the transaction. You need to establish a "Gen. Prod. Posting Group" on your Item or G/L used in the transaction. You will likely need to delete and re-enter the line in question to pull in the new value from the related record.
Once you have updated the record, it would be a good idea to check other similar records to ensure they are properly setup. Once your data has been fully setup this warning message will no longer appear.
P.S. This is a large reason why companies use Item Templates is to ensure the proper values are setup based on the type of Item being created.
