You can only delete journal entry rows in the detail or journal entries in the header if the journal entry has not been posted. 

To delete a row or an entire posted journal entry, you must unpost the journal entry first.  

‌A warning message appears if you try to delete a row or entry that's been posted.

Delete a journal entry

To delete a journal entry:

  1. Navigate to the Journals module.
  2. Select the appropriate unposted entry or entries from the header.
  3. Select Delete from the header toolbar.

Delete a row

To delete a row:

  1. Select the appropriate unposted entry from the header.
  2. Select the appropriate row or rows from the detail.
  3. Select Delete from the Detail Toolbar.