SmartConnect to D365 Business Central: Failed to create record. Unable to call service: You may not enter numbers manually

In Business Central, search for 'No. Series'

You'll see a form with the same name

Scroll down to the 'code' with the same name that you see in the error. Check the appropriate Checkbox

Edit 11/2/2018: I never got this working with the nativeInvoicingSalesInvoices web service. I switched to the Sales Invoice web service and it started working