Custom variable

I am using Shipment Tracking
https://woocommerce.com/products/shipment-tracking/

They are using Custom meta fields
‘_tracking_provider’
‘_custom_tracking_provider’
‘_tracking_number’
‘_custom_tracking_link’
‘_date_shipped’

we wrote _tracking_number in Custom variables section of setting page
then we add %_tracking_number% (or _tracking_number) in Order Completed message section, but in sms it is not showing tracking number instead its shows _tracking_number

Please resolve this issue