How do I cancel a pending purchase

Speak to Apple Support.

Direct link to Apple ID Support options -

https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0998&symptom_id=20198

World-wide Apple Support -

Contact Apple for support and service - Apple Support

M1 Mac mini, macOS 14.3.1;  Watch, series 7 GPS/cellular, watchOS 10.3.1; iPhone 12 Pro, iOS 17.4; TV 4th Gen, tvOS 17.3; HomePods, audioOS 17.3 and Airport Express w/ Airplay 2

Click on this link to read how to 'Request a refund for apps or content that you bought from Apple' -->

Request a refund for apps or content from Apple - Apple Support

"If the charge is pending, you canʼt request a refund yet. After the charge goes through, try to request a refund again."

To request a refund for apps or content that you bought from Apple, click here → Request a refund for apps or content from Apple - Apple Support

If you wish to contact Apple about this, my inclination would be to try "Billing and subscriptions" support, click here ➞ https://getsupport.apple.com/?caller=cups&PGF=PGF63006&category_id=SC0060