If you have enabled a Private Encryption Key through the "Security" settings in your Backblaze, then this key would be required in order for you to restore your data. A Private Encryption Key exists to add an additional, optional, layer of security to your backup.
Setting a Private Encryption Key ensures that no one but the person who created the code will have access to your data. Meaning that even if your login credentials are compromised your data would not be able to be accessed by anyone who does not have the key that you created, including the team at Backblaze. Because of this added layer of security, your Private Encryption Key must be supplied in order for you to create a restore.
For information on what happens if you misplace the Private Encryption Key, please view our article: What can I do if I forgot my Private Encryption Key.
For additional information about the Security the Backblaze employs to protect your data, please also see our article: Can you tell me more about the encryption Backblaze uses?
Please note as mentioned before, that adding a Private Encryption Key is an optional feature, and you are by no means required to implement it should you not wish to have to supply a key any time you create a restore of your data.