You have to cancel a subscription yourself, we can't do it for you.
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
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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)
Also read the condition of CamScanner - PDF Scanner App.
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UNLIMITED ACCESS MEMBERSHIP SUBSCRIPTION
–You can subscribe to get unlimited access to all features of the scanner app.
–Subscriptions are billed weekly, monthly, quarterly, or annually at the rate based on the subscription plan.
–Payment will be charged to iTunes Account at confirmation of purchase.
–Subscription renews automatically unless auto-renew 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. The cost depends on the selected plan.
–Subscriptions may be managed by the user, and auto-renewal may be turned off by going to the user's Account Settings after purchase.
–Any unused portion of the free trial will be forfeited when the user purchases a subscription.