To do that you first have to make sure that Request Quote is disabled for all products. 

Please follow the steps to check if the Request Quote is disabled for all products.

After that Go to Apply Products section in the sidebar menu and select the product you want to active for Quote.

Then activate the product for Quote and customize the quote option (if applicable).

Your selected product will be available for Quote in your store.

NOTE:- If you need any help or faced something wrong then please contact us by creating a ticket here or by mail  here