First, click More views in your Customer Portal Settings to reveal additional options. This section is useful if you prefer having the Manage Subscription button within your customers' accounts display in a custom location.

Normally, in cases where the button does not display automatically in your Shopify theme, you can add the button in your Shopify Theme Settings.

When you click to Customize, you will see your website and a panel to the left where you can make edits. In the website, click to log in to an account to view account details.

From here, click to Add block, flip to the Apps tab, and click Manage Subscription Link.

The button will now display.

If you wish to place this button in a custom area, you will need to add the code snippet you see in your Customer Portal Settings to the sections/main-account.liquid file in your Shopify store's theme files, accessible from your theme settings as well.