If you ever come across the below error message: 

This means that you have already sent the invoice via email to the client. There are permissions available that will allow users to un-approve invoices that have already been sent but you will need to contact your system administrator or site super-user to give you this permission:

If you are unable to get this permission you will have to get someone who does have the permission to un-approve the invoice for you, but if you are given the permission moving forward you will get a warning like the one below when you attempt to un-approve an invoice that was sent via email.

Simply click OK on the pop-up warning and you will un-approve the invoice.