It is not possible to cancel a payment to an existing Venmo account. Once you send a payment, the funds are available to the recipient right away. If you need the payment returned, please have the recipient send you a payment for the same amount. Upon return, you can transfer those funds from your Venmo account to your bank account.
Venmo Support can only reverse a payment if the recipient gives their explicit permission, their account is in good standing, and they still have the funds available in their Venmo account. Venmo Support cannot reverse a payment at the sender’s request.
If you sent a payment to a phone number or email address that has not been registered with a Venmo account yet, you can take back the payment in the “Incomplete” section of the Venmo app. See this article for more details.
Note: Payments sent via iMessage work a bit differently. Check out this article if you sent your payment via iMessage.