"Journal Name may not be changed when vouchers have been created." error message when entering General Journal
The particular number sequence can be found by:
1. Click General Ledger, click Setup, click Parameters and then look on the Number Sequence Tab. The Number Sequence you will be looking for will be the Number Sequence for Journal.
2. Right-click in the Number Sequence Code column and select the Main Table.
3. In the Number Sequence Window, click the List Button and delete any lines out of the window to correct the issue.