I have found my Citi Executive World Elite card to be a huge PIA when it comes to charging certain items. It is impossible to use for purchasing SMGCs since it will decline any such purchase (no matter the amount) and even after the decline is cleared, is now ineligible for use in SM due to their no use for 24 hours after decline rule. I suspect that Venmo is similar in that trying to attach the card to Venmo produces an automatic decline by Citibank after each charge which requires manual clearing by the card owner. Venmo probably does not want to deal with that on a routine basis, and therefore has simply banned the card for their site.

I have found that it is very difficult to MS using Citibank's card, so I don't use it for that purpose.