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When transaction is created using Shopify POS, customer email address is not required (contrary to Web Checkout where email is required field). This might create situation when App Gift Card is sold on POS but there is no email address attached to order and gift card can't be sent.
If you are using Shopify Point of Sale, make sure App Gift Card is unavailable on your POS sale channel to avoid accidentally selling it there.
Your Shopify Gift Card should be only gift card available there. Shopify Gift Card offers an option to add recipient email address in Shopify POS application when checking out.