Can I convert a quote into an invoice?

No, you cannot convert a quote into an invoice but you can generate an invoice from a quote. 

Follow these steps to generate an invoice from a quote:

1. Click the Menu icon.
2. Go to Sales. 
3. Click Quotes.
4. Select and open the quote from which you want to generate an invoice.
5. Click the +New button located in the top section of the quote.
6. Select Add or Create.
7. Select Invoices.
8. Add/change the necessary details in the invoice.
9. Click Save.