Waitlist button should work on Subscription Products
Please allow the Waitlist Extension to work with Subscriptions.
Currently, the Waitlist Extension works with products in your WooCommerce store that run out, so customers can sign up to be notified when the product becomes available again, etc. Unfortunately, this functionality does not apply to WooCommerce Subscriptions. So if a subscription has a limited number of subscribers, wanna-be subscribers are not able to get on the waiting list.
WooCommerce Waitlist is compatible with WooCommerce Subscriptions.
Neil Pie commented
Compatibility with WooCommerce Subscriptions was added some time ago :)
2017.02.02 - version 1.5.0
* Added support for WC Subscriptions