This means that the domain you are trying to add is already tied to another Cloudflare account. This generally happens to users that signup through a hosting provider, then try to signup through a different hosting provider with a different account down the road. Removing the domain from the old account will allow you to add it to another account and steps are included below.
To remove the domain from the old account:
1. Login to the Cloudflare account that the domain was originally added to.
2. Go to Settings for the domain.
3. Go to DNS settings.
4. Remove the connection to the hosting provider using the red button.