I created a new wallet file and set a password on a computer without internet access. But when I want to unlock and extract the private key, it displays "Method not found (core-32601)". I unlock it like this: walletpassphrase password unlock duration (seconds)
The format should be like this: walletpassphrase "PASSWORD" 600
Replace the PASSWORD with your own.
This is how I do it, but it doesn't work! I unlocked it without synchronizing the blockchain.