It is possible to delete an expense from a report if you added it in error, even if you have already submitted the report for approval.
How to remove an expense
Ask the user who approves your expenses to reject the report. They can do this by clicking their Review tab within the Expense Reports folder, viewing the relevant report, and selecting Reject.
The report will then move back to your Reports tab, where you can then Re-open it.
Once re-opened, you can remove the incorrect expense from the report by clicking the trash can icon on the right-hand side. That expense will then be available in your Inbox again, ready to be added to a new report!