How to cancel a pending subscription of Snapchat premium in App Store?

You cannot cancel pending items. You have to wait for them to complete, then cancel them. If this is going to be a problem, you will have to contact Apple. For help with "Billing and subscriptions" support, click here ➞ https://getsupport.apple.com/?caller=cups&PGF=PGF63006&category_id=SC0060

Sadly, waiting for the pending purchase to be complete, as suggested, means that you will have to ask Apple for a refund. Apple very rarely gives refunds, as witnessed by scores of topics on the subject of refused refunds in this Apple Support Communities forum.

Contact Apple Support and request that the pending purchase be stopped.

M1 Mac mini, macOS 15.1;  Watch, series 10 GPS-only, watchOS 11.1; iPhone 16, iOS 18.1; TV 4th Gen, tvOS 18.1; HomePods, audioOS 18.1 and Airport Express w/ Airplay 2

Once you double click the side button with the amount showing on the screen, you will be charged for the subscription you have authorized. The charge may show as pending for a day or two, but there is no way to cancel it at that point.