Answer:

The eCard design might be discontinued or you haven't selected a card design yet.

Discontinued eCard/Invitation Designs: Older eCard designs are sometimes retired and are no longer available when you create a new eCard, edit a saved eCard or copy a sent eCard. You must select a new card design before you can send the eCard.

No Card Design Selected: Since you can compose and/or address an eCard before selecting a design, this message will appear to remind you that you need to select a card design before the eCard/invitation can be sent.  

Discontinued eCards that Have Already Been Sent: The eCard/invitation design can still be seen as a sent eCard from your account. Any links to your sent eCards/invitations will still work properly and show the retired eCard design.