When you try to post a journal entry to selected G/L Account, the following error message prompt up:
Gen. Posting Type must be specified.
This problem occurs because you have specified Gen. Prod. Posting Group for the G/L Account selected. To solve this problem, you can either:
1) Remove the Gen. Product Posting Group from the journal lines
2) Remove the Gen. Product Posting Group from the G/L Account Card and then reselect the G/L Account in Journal lines.