Where to do I find my recovery key? I just chose FileVault encryption and it said the Recovery key has been generated..

• If you're using OS X Yosemite or later, you can choose to use your iCloud account to unlock your disk and reset your password.*
• If you're using OS X Mavericks, you can choose to store a FileVault recovery key with Apple by providing the questions and answers to three security questions. Choose answers that you're sure to remember.*
• If you don't want to use iCloud FileVault recovery, you can create a local recovery key. Keep the letters and numbers of the key somewhere safe—other than on your encrypted startup disk. 

Reset your password or change your FileVault recovery key

If you forget your account password or it doesn't work, you might be able to reset your password.

https://support.apple.com/en-us/HT204837

When you first set up FileVault in the Security & Privacy system preference pane in the FileVault tab, one of the steps asks you whether you want to use your iCloud account as a way to unlock your disk and reset your macOS account password if you can’t find your recovery key.

If you choose iCloud, the recovery key isn’t stored loosely in iCloud Drive or as a file, but it’s tied into behind-the-scenes account information that Apple maintains. It’s fully encrypted in such a way that even Apple doesn’t have access to the unencrypted recovery key data, but Apple can deliver the encrypted recovery key to your Mac if you need to reset your password. You never see the recovery key nor have to enter it in this configuration. (The process is a little involved: Apple describes it in the section “Reset using the Reset Password assistant (FileVault must be on)” in this support document.)

If you choose the other path, where FileVault generates a recovery key and displays it, you need to make sure and write it down or enter it electronically, and store it securely in such a way that you’ll have access even when your Mac can’t be booted. I use 1Password’s secure notes for this purpose, but any method of storage that’s reliable, secure, and accessible will work.

A good strategy would be to set a quarterly reminder to look for your recovery key (and other important passwords and keys you have to store in the same place). If you can’t find it, disable FileVault in macOS and re-enable it. This will take a while, as the entire drive is decrypted and then re-encrypted, but macOS generates an entirely new recovery key, which you can then more carefully note again.