First, the most common reason why your upsell may not be showing on the storefront is that the upsell product isn't available on the storefront.
Track Quantity
If “Track quantity” is enabled, the product must have available inventory.
Check the Available column in your Shopify Admin:
If the number is 0, the add-on won’t appear.
Shopify uses four different inventory states. You can learn more about them here.
For single-variant products:
Scroll to the Inventory section to view stock levels.
For multi-variant products:
Go to the Variants section.
Click on each variant to view its inventory settings.
Active Status VS. Draft Status
Make sure the product is set to Active.
If the product is marked as Draft, it will not display as an upsell.
You can verify this in the Products tab of your Shopify Admin.
Look at the Status column next to the product name.
Online Store Sales Channel
Ensure the product is available in the Online Store sales channel:
Go to your Shopify Admin → Products tab.
Select the product.
In the Publishing section (top right), look for a green dot next to Online Store.
If it’s not there:
Click the three-dot icon in that section.
Select Manage Sales Channels.
Add the product to Online Store.
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