If the email address you use with Checkout 51 is already connected to a Venmo account, but you have not verified this email address with Venmo yet, Venmo will not deposit the funds into your account until your email has been verified.
If your Venmo account is connected to a different email address than the one you use for Checkout 51, you can add your Checkout 51 email address as a secondary email on your Venmo account and once you have verified your email, the funds will be deposited.
For more information on verifying your email address or adding a second email to your Venmo account, please see here.