Error: Account code '###' has been archived, or has been deleted. Each line item must reference a valid account.
When this error occurs it means that one of the Xero accounts that an Expense Category has been mapped to in your integration settings has now been archived in Xero.
To fix this problem you can either:
Update the Expense Category mappings on your Xero Integration page so that the archived account is no longer used
Unarchive the account in Xero
Once the problem is resolved you can resync the expense and it should flow in Xero correctly.