The Venmo system can decline purchases arbitrarily, based on supposed security flag and will not disclose what that may be via private notice or through customer service team assistance. Imagine purchasing with friends and agreeing to share payment in this way only to be declined without reasonable indications or avenues for reconciliation. Not a good system!
Customer service cannot help when needed in this type of case.
“This doesn't necessarily mean that you've done something wrong, as the declines are automated by our system. I'm very sorry about the inconvenience here, but at this time we are not able to override these declines. For security reasons, customer facing teams like myself are not able to see the exact reason for the decline, but in some cases you may be able to try the purchase again later.
Unfortunately due to the way our system is set up for security purposes no one is able to see the specific reason. I know it's not ideal, but again you might be able to try again later, since most of the time these declines do clear up with time.
Users are not notified of specific reasons by our system.”