The Quantity selector won't show if the quantity breaks add-on is enabled. This happens because the quantity breaks add-on conflicts with the quantity selector. It is not an issue, it is done on purpose to avoid any quantity issues.
For the Quantity selector to show, check if the Quantity breaks add-on is enabled, so you can disable it.
How to check if the Quantity break add-on is enabled
From Shopify admin, go to the Online Store.
Select your Debutify theme, and click on Customize.
Click on Theme settings.
Click on Debutify Add-ons.
Scroll to Quantity breaks and uncheck the box to disable the add-on so the quantity selector can show on the product page.