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 Main Menu.
2. Go to Sales. 
3. Click Quotes.
4. Select and open the quote from which you want to generate an invoice.
5. Click the More icon in the top-right corner.
6. Select Invoices.
7. Click Yes to copy the data from the quote to the invoice. Creating Invoice window opens.
8. Fill in all the mandatory fields.
9. Click Save. A successful message appears with the invoice link.

To know more about Quotes, click here.