Why does Shopify show inventory on hand but make the product unavailable at checkout?

“On hand” doesn’t mean “sellable.” In Shopify it includes units that are available, committed to existing orders, or marked unavailable by an app, safety-stock rule, damage hold, etc. Check the exact variant—not just the product total—and compare its Available, Committed, and Unavailable numbers at each location. Then check whether a location with Available stock can actually ship to the customer’s address. Shopify only lets checkout complete when stock is at a location that delivers to that shipping zone, so this often shows up with multiple warehouses, shipping profiles, or markets. If the item is meant to be purchasable without stock, the variant’s “Continue selling when out of stock” option is the separate setting to review.