The contact with the specified contact details has been archived. The contact must be un-archived before creating new invoices or credit notes.
If you receive the above error, it means you have archived (or merged) the contact of the invoice in Xero.
To resolve this error, you’ll need to restore the archived or merged contact.
Some information you should know about contacts in Xero:
You can't delete a contact from Xero, but you can archive a contact you no longer do business with.
You can view archived contacts, including their transactions, in reports. Transactions, created for the contact, won't be deleted. This includes any outstanding invoices or bills.
You can restore a contact you've archived or merged, and their activity details, as long as another contact doesn't exist with the same name. Once a contact is restored, you can select them when entering transactions.
To restore a contact you've archived or merged in Xero
In the Contacts menu, select All contacts.
Select the Archived tab.
Click on the contact to open their details, then click Restore.
Click Restore to confirm.
For more information on this error, please see the Xero support page: Xero Central