How to get the recovery key on MacBook Air?

A FileVault recovery key? If so, you may or may not have one. See the following from Use FileVault to encrypt the startup disk on your Mac - Apple Support

Choose how you want to be able to unlock your disk and reset your password, in case you ever forget your password:

• 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.

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