how can i import sales Invoice
Short answer - you can’t
Longer answer - what sales invoices are you trying to import? it might be possible to handle them differently if you gave more details about the business, customers and workflow
And where would you import them from? Sales invoices require considerable information that is not available outside Manager.
Are you referring to importing an invoice format? You cannot do that. You must create a custom theme to achieve different layouts. See https://guides.manager.io/10368.