If you're trying to convert an expense transaction into a journal entry directly, this option is unavailable in QuickBooks Online (QBO).
You'll want to delete the expense and create a journal entry. To delete the expense, just open the transaction, click More in the footer and select Delete.
Here's how to create an journal entry:
Go to + New.
Select Journal entry.
Enter information in the memo section so you know why you made the journal entry.