can't find manage subscriptions in account settings for ios app store

Not sure which instructions you followed so will give the following just in case. The following from Manage your auto-renewing subscriptions - Apple Support just worked for me with the minor differences of instead of the screen showing "Manage App Subscriptions" it shows a section titled "Subscriptions" in light gray and centered below that the word "Manage" in blue.

Use your iPhone, iPad, or iPod touch to manage subscriptions

1. From the Home screen, tap App Store.
2. Tap Featured from the menu at the bottom.
   
3. Scroll to the bottom of the page and tap your Apple ID. If you don't see your Apple ID, tap Sign In and enter your information. Then tap your Apple ID.
4. Tap View Apple ID.
5. If prompted, enter your password and tap OK.
6. On the Account Settings screen, scroll down and tap Manage App Subscriptions.
7. Tap the subscription you want to manage, then choose a subscription category.
8. Use the options to manage your subscription. You can choose a different subscription period, or turn off Auto-Renewal.

I had this problem too and it ended up being that Apple did not have any record of subscriptions for me because I started my subscription through the Google play store on my previous phone. So, even though I'm using the app on iPhone, I had to go to a computer to log into the Google play store and cancel my subscription. I think the Subscription button only shows up on iphones when you start a subscription to an app through the Apple App store.