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.
bitaddress.org does not accept 12 word seed phrases in the normal sense. If you entered them as a brain wallet, then it has simple hashed those words and produced a private key and address from the hash. You will not be able to import those words in to another wallet and recover the same private key.
If you have the private key printed on the paper wallet, then that is what you will need to use. Follow Maus0728's link above to download and verify Electrum, import the private key to an Electrum wallet, and then send the funds to your Coinbase exchange account to be sold.
I downloaded Coinbase Wallet on the Apple store, and when I run the app, it asked if I had a 12 word seed.
Note that the Coinbase Wallet App and the Coinbase Exchange are two completely separate platforms. If you send the coins to the wallet app you will not be able to sell them on the exchange. Make sure you transfer them to your exchange account.