Whenever you create a quote, you have two options for customers.
1. Existing Customer.
2. Guest Customer.
Who has an account in your store is an Existing customer. If you want to quote them simply select the customer from the customer list and create/send the quote.
Who doesn't have an account in your store is a Guest Customer.
If you want to quote a customer who doesn't have an account in your store then you can create the quote by simply entering the Guest Customer email address.
To create a quote with Guest Customer, Click on the Guest Customer button
Enter the guest's customer email.
Then create the quote.