Yes, you need a Shopify Online Store.
For you to start working with Sesami, you need to select one of the 3 Shopify Plan and create an Online Store.
The Sesami app isn't compatible with Shopify Starter!
The Starter plan is designed for selling on social and messaging apps. It isn't like one of the 3 main Shopify plans, as it doesn't provide an e-commerce website.
After signing up for Shopify, you simply install Sesami on your Shopify store by entering your Shopify store URL: yourstore.myshopify.com on our application sign-up page.
Alternatively, you can install Sesami from the app section of your Shopify store: https://help.shopify.com/en/manual/apps/working-with-apps#installing-an-app