How do I find my recovery key when I have turned on Firevault?

Then I can't help you. I am no expert with FV2. You can log onto your iCloud account with your Apple ID and password. If it's there then you may be able to find it. Otherwise, contact Apple Support for further help if no one else posts help.

Contacting Apple Customer and Support Service

1. Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
2. Primary support help at Contact Support.
3. To contact product and tech support: Contacting Apple for support and service including international calling numbers.

It was provided when you set up FileVault. You were supposed to write it down. Without it you must hope you sent a recovery key to Apple. You have to call Apple to get it. You could try: How to bypass FileVault when you restart your Mac | MacIssues, and, if that works, you may be able to turn off FileVault.

You should be good to go then. I’ve had the same thing happen to me and it turns out it only occurs on APFS volumes. You can test whether your Apple ID can unlock your startup disk by booting Recovery Mode, executing resetpassword in Terminal and selecting I forgot my password. If your Apple ID works then you will be able to reset your password.

Sorry, I should have clarified what I was saying. If you choose to store your recovery key in iCloud, you are never shown the recovery key itself. It is immediately stored away in iCloud for you. To reset your password, you would need to use the Password Reset Assistant as I mentioned in my earlier reply. Your recovery key will automatically be retrieved from Apple's servers when you sign in with your Apple ID, and will be used to temporarily unlock your startup disk so that you can reset your password.

Choosing not to store the recovery key in iCloud, however, will result in your Mac showing you the recovery key that you need in order to reset your password.

At any given time, it is not possible to locate the recovery key on your Mac. However, you can destroy the old recovery key and create a new one using this command while logged in as an administrator:

sudo fdesetup changerecovery -personal

Enter your administrator password when prompted. Terminal will not show any characters as you type it. Next, you'll need to enter the name of a user that can unlock FileVault and it's password. Once you've done that, Terminal will show you the new recovery key. Be aware that by doing this, you could invalidate your Apple ID from unlocking FileVault.