Simply piano App has charged me annual fee which without my consent. I requested to get the refund but refused. And, the app does not have un-subscription button. Who can help me?

1.

If the invoice has to do with an app that has a subscription associated with it, see -> Delete apps on your iPhone, iPad, and iPod touch -> Delete apps on your iPhone, iPad, and iPod touch - Apple Support

• If you delete an app, it doesn’t cancel in-app subscriptions that you might have. While deleting an app, you might see a Cancel Subscriptions option. Learn how to cancel your subscriptions.

If you don't want to continue a subscription you need always to cancel that subscription in time, see the following Apple articles:

If you want to cancel a subscription from Apple

If you want to cancel a subscription from Apple - Apple Support

If the subscription that you want to cancel is not listed

If the subscription that you want to cancel is not listed – Apple Support (UK)

2.

If you have received an invoice from Apple, on every invoice from the Apple Store and/or iTunes Store always states what you have purchased (and/or concerns a subscription) and the amount.

Tap on Report a problem on the invoice.

Or try to ->  Request a refund for apps or content that you bought from Apple.

Request a refund for apps or content you've bought from Apple – Apple Support (UK)

• Some purchases from the App Store, iTunes Store, Apple Books or other Apple services may be eligible for a refund. 

If you have requested a refund, you must wait until Apple has approved. Then you will receive a message from Apple.

Refunds are not guaranteed. For more information, see the Terms & Conditions.

3. 

Refunds are not guaranteed. For more information, see the Terms & Conditions.

If Apple has notified you that it is not eligible then so be it.

We can't help you, sorry, we are also users of Apple products & services like you. 

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

4.

Check also here, see -> How the Subscription Works:

- All app users can play two free courses. Upgrade to a premium subscription package at any time.

- Subscribers will receive full access to ALL courses. New courses and songs added every month!

- All recurring subscriptions will auto-renew unless auto-renewal is turned off at least 24 hours before the end of the current period.

- Account will be charged for renewal within 24-hours prior to the end of the current period, at the cost of the chosen package.

- Any unused portion of a free trial period, if offered, will be forfeited when the user purchases a subscription to that publication, where applicable.

- Subscriptions may be managed by the user and auto-renewal may be turned off by going to the user's Account Settings after purchase.

- Your subscription is for your iTunes account and can be used on your iOS device.

- Payments will be charged to your iTunes account at confirmation of purchase.

- Subscriptions cannot be cancelled during the active subscription period.

C8826 wrote:

And, how to un-subscribe apps without any un-subscription button. Urgent.

This support article explains how to cancel a subscription from Apple ➔ If you want to cancel a subscription from Apple - Apple Support

Notes:

• You might need to scroll down to find the Cancel Subscription button. If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew. An "Expires" date (instead of "Renews") means the current subscription will just run until then. "Resubscribe" is just in case you change your mind about having it end.

Once you have successfully canceled the subscription, follow the directions in this support article about how to request a refund for apps or content that you bought from Apple ➔ Request a refund for apps or content that you bought from Apple - Apple Support