If there is no 'Cancel Subscription' button, this app has already been unsubscribed from.
If you cannot find 'Subscriptions' in the Settings app, you should try to find the button in iTunes & App Store. Click your Apple ID, view it, scroll down to Subscriptions. Option 3
You can also study Apple's help articles
to get extra information and guidelines.
How to check out subscriptions and their status in iTunes using Mac or Windows PC?