On an offline computer, I used a copy of bitaddress.org to enter a brainwallet passphrase of 12 words and the private key and public key were displayed. I printed that offline, and keep the info safe until now. I'm confident I have the public and private key. Just trying to move the coins safely to coinbase.
A brainwallet with 12 words is
not a seed/recovery phrase. That is the source of your problem.
To recover your bitcoins, go to bitaddress.org and use the 12-word brain wallet to generate the private key again. Then sweep that private key into a wallet of your choice.