Assuming you have the Buy Button sales channel enabled (check Settings → Sales Channels in the Shopify admin), this message will appear in one of two cases.
The first, is that the product itself is no longer available. This may indicate a syncing issue with Landed, and can usually be resolved with a manual resync (‘Resync Shop’ in the bottom right corner of the Landed admin).
The second case, is when a product has unticked the ‘Buy Button’ under the ‘Visibility’ options for that product in Shopify. Make sure this box is ticked for all products available in your Landed stores.
If you’ve tried both of the above, and you are still seeing this message on some products, please get in touch with us, and be sure to include any observations you can make about the affected products!