This issue can occur when you're tracking inventory across multiple locations, and one of your locations isn't designated as a fulfillment location.

To resolve this, you'll need to transfer the inventory in Shopify to a location marked as a fulfillment location or designate the location with inventory as a fulfillment location.

1. Navigate to Shopify Settings and select "Locations" from the left navigation menu.

2. Choose the location you wish to update, then scroll down to locate the below option:
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3. Tick the checkbox inside the highlighted area, then scroll back up to save the settings.

4. Once done, you are good to go!