Hi @zyzl91,
Thank you for reaching out and sharing your question with the Klaviyo Community.
The tracking number is a part of the event data sent from Shopify to Klaviyo through the Fulfilled Order event, rather than the Placed Order event. This means that only flow emails triggered off of Fulfilled Order are capable of displaying the tracking number.
We have a prebuilt Shipping Confirmation Flow that already has the tracking number pre-populated in the email. You can build this flow if you navigate to the Flows Tab > Create Flow (top-right) and select Shipping Confirmation.
If you click into the flow email, you can preview the email using the Preview icon on the left-hand side to view all information pertaining to the Shipping Confirmation email, including the tracking number.
Thanks and have a great day.