If you see this message, it means your child already has access to this product on their account. You cannot activate the same product twice on Cambridge One. 

This may happen if:

• You already used a different code for this product
• You are trying to add a second code from the same product before the first one has expired.

Once their code expires, you can activate a new code for the same level.
If you're not sure when the code expires, you can check the expiry date on either your account or your child's account: