If the card needs to be activated with the subscription it needs to be done at an OV-chipcard machine. And if the Kids Vrij was ordered on a new card it doesn't need to be activated, since it is already on the card in that case.
But the message Vesna received says she needs to activate the MijnNS account and the invition for that has expired. So these are two different things.
This activation has to be done on a TICKET machine-from NS-those yellow things in any station by NS. Thats the way the subscription lands on the card. Use ´ophalen bestelling´ in dutch. Thus an online message like you show cannot be the reason.
The invitation to start a Mijn NS-account can expire or you can get that error because it is already activated. Our costumer service can check this out and help you with the account.
Did you activated by a ticketmachine?