If you are unable to find the client's CVV number on their form response, it's likely that you have not required the form to collect that information.
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To collect a client's CVV number when sending them a credit card authorization form, make sure to enable that option in the form. To do so, click the pencil icon next to your card authorization question within your form.

Then change the Collect CVV option to Yes.

Once that's enabled, the form will require the client to add their CVV in order to complete the form.