How to add a track button in woocommerce order completed email?

I set up the settings as the manual book said. I’ve turned on the ‘Include tracking in WooCommerce email’ to yes.
And I have input the codes {tracking_list}, {tracking_table} to the Tracking content.
And then, I tried to place an order to test the function. Unfortunately, the email comes without the tracking button. There is only the tracking number in it(It looks like a default content for WooCommerce).
So that means WooCommerce Orders Tracking does not work.
What should I do if I want the tracking button to be shown in WooCommerce order completed email? Because I want my customers can track their orders in an easy way. A button for tracking orders in the email is necessary.
Please Look at the following images: