When editing an invoice you're unable to click the Add an Invoice Item button.
Once an invoice has been submitted as a claim it cannot be edited or changed.
If it's been Submitted you need to mark the claim as "Failed/Rejected" first. You will then be able to make changes.
If it's in Claims Awaiting Submission you will need to remove it from that section by marking the box next to the claim, then click the Remove Selected Claims button.
See more here on correcting submitted claims.