I see you're unable to log in to your MacBook Air because you're unable to change your password, due to an encrypted startup disk. I understand needing to regain access to your Mac, and I'd be glad to help.
Take a look at the following articles with some steps that can help. If you're unable to unlock the encrypted start up disk, you'd need to visit an Apple Store or Apple Authorized Service Provider.
Use FileVault to encrypt the startup disk on your Mac
Change or reset the password of a macOS user account
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