As of the Fonteva 20Spring release, the Invoices object has merged with the Sales Order object. 

Invoices and Invoice Lines no longer get created as of the date your organization updated to Fonteva 20Spring. Rather, Sales Order will get converted to Invoices.

For more information on the new architecture for Invoices, please refer to the Order Enhancements document attached to the Fonteva 20Spring Release Notes.

Your organization will likely still have Invoice records from before the update. Documentation on how to work with pre-Fonteva 20Spring Invoices is available here: