Check that:
the app embed is on in the theme you’re viewing,
the field is turned on,
its product rules include this product.
If your theme puts Add to cart somewhere unusual, add the Upload box block to the product template in the theme editor instead.
Still stuck?
Open the app in your Shopify admin, go to Help and click Chat with us, or email [email protected] with your store’s myshopify.com address.