If the number of the invoice you're trying to sync already exists in Xero, the sync will fail as you can't have more than one invoice with the same number in Xero.
For example, if in Xero an invoice with number 1234 already exists, and you set the Invoice ID for an invoice in Dynamics 365 to be 1234 (i.e. the same as a pre-existing Xero invoice) then when the Dynamics 365 invoice is attempted to be synced to Xero, it will fail because that invoice number already exists in Xero.
This can happen if you have set your connector to use the Dynamics 365 Invoice Numbers (Use Xero invoice number is set to No in Stellarise Connector Settings)
To fix this, you can either:
Void or Delete the invoice with the same number in Xero
Change the invoice number in Dynamics 365 to one that doesn't exist already in Xero
Change the connector settings so that Use Xero invoice number is set to Yes so that the next available Xero invoice number is used for this invoice (instead of 1234) - see screenshot below: