I want to cancel my purchase of iTunes

iTunes is free. Are you referring to his? -->

Manage your Apple Music subscription - Apple Support -"Learn how to change from an Individual membership to a Family membership, turn off automatic renewal, or join Apple Music later." Turn off automatic renewal and your membership will finish at the end of the current subscription period. --> https://support.apple.com/HT204939

The term "iTunes" means an application on a computer or an app on a device. It does not mean a purchase. If you are referring to seeing it on a bill it, it is not for "iTunes" it is for the iTunes or App Store. Click on the link to read what to do "If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement" --> https://support.apple.com/HT201382 - Does not mention purchases or subscriptions made by another family member as part of Family Sharing which might be charged to the organizer's card but will not appear under the organizer's purchase history or subscriptions. Ask family members about those or check your receipts.

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