unfortunately, those two ad accounts do not have the card ending in 0621 on file. The reason I was asking for an ad account that already has the card ending in 0621 on file is because the card had been added to too many accounts. ****INTERNAL ONLY**** a credit card can only be on 10 accounts before a user is blocked from adding them to an 11th account. In these situations, a user will have to write in to us for us to "whitelist" the card, meaning that we essentially approve the card to be added to more than 10 accounts. That's why your client was unable to add their card initially.
Luckily, I was able to find some other accounts that your client's card was on (AID 444906159047575, for example), so I was able to manually whitelist the card. Your client should be able to add it to their two new accounts now.
Since whitelisting is an internal term, we're not allowed to share this information directly with our clients, but you're more than welcome to just tell them that they shouldn't have any further issues adding their card ending in 0621 to their ad accounts.