In Desktop version 19.2.78, in the Sales Invoice tab, after choosing an existing transaction, clicking View & then clicking Copy To, the top option displayed in the dropdown is named New Purchase Invoice. I believe it should be named New Sales Invoice; is that correct? See screen shot below.
If you want to generate a new sales invoice from an existing one, choose Clone. Copying only allows you to create other types of transactions. Read the Guide: https://www.manager.io/guides/7262.