The most common reason for errors is that the contact already exists in Xero, but the invoice did not have a Xero ID in SimpleClinic. You can modify a client's Xero Contact ID by going to the client record. The current Xero ID will be displayed at the bottom of the client record above the Archive and Save buttons. By default this record is locked, click the pencil button to edit.
Once you have accepted the warning the field will be editable. Don't forget to hit Save after making changes.
You can find an existing contact's correct Xero ID in Xero by logging into Xero and select Contacts -> All Contacts from the top navigation menu. This will open a list of contacts with a search bar at the top. You can search for your contact here.
After you have found the client in the Contacts list click on the name and the Contact record will open. The Xero ID is in the URL in your address bar. See below. Copy and paste this into the Xero ID on the client record in SimpleClinic. Any future invoices for this client will be linked to the new Contact id.