Thank you for your reply. So the way I add a tracking number to a single order is that I open the order through Dashboard > Woocommerce > Orders > and inside the order details, there is a section to attach the order tracking number and choose delivery service provider. There I type and add a custom tracking number to my order. When I click on that number and it takes me to my delivery service provider but when I enter my created code it shows an error. This sums up my way of adding code.
If you have any more questions please let me know.
Waiting for your response.