Hello mvcrev,
It sounds as though you're being asked to enter a recovery key created when enabling FileVault on your Mac. If you're entering a 28-digit recovery key, that is likely the recovery key associated to your Apple ID. The key associated to your Mac and FileVault would be a 24-digit key. You can learn more about the recovery key associated to your Apple ID in the following article.
How to generate a recovery key
If you no longer have access or forgot the recovery key associated to your Mac and FileVault, you would want to turn off FileVault in System Preferences > Security & Privacy > FileVault, click the lock icon if it appears locked, then click Turn Off FileVault. Once turned off you'll be able to turn FileVault back on to generate a new key. This will also disable all older keys associated FileVault.
You can find this information in the "Reset your password or change your FileVault recovery key" section of the following article.
Use FileVault to encrypt the startup disk on your Mac
Take care!