I purchase an app from the app store but accidently purchase a subscription, although it said the purchase of subscription did not go through, but I was still charged

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)

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.

but I was still charged.

If you have received an invoice from Apple, it will be stated on the invoice for what you have to pay.

Tap on Report a problem on the invoice.

You can also 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)

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.