The address book is already connected to another Fullcontact account. Please contact support for assistance.
This notification usually means that the account you're trying to connect is already associated with another FullContact account. Since one contact source can only be associated with one FullContact account, you aren't able to connect it twice.
If you have created multiple accounts accidentally, we simply suggest deleting one of them. After that, you should be able to connect the address book which returned this error without any issues.
To delete the accidentally created FullContact account, go here -> Close Account.