How can I retrieve the mnemonic words from my bitcoin core wallet? I studied with Aaron Jaramillo's Libbitcoin tutorial.
And I have a question regarding mnenonic word.
In that tutorial, I should use testnet for making raw transaction.
And I should make wallet from mnemonic words from my testnet wallet.
But I cannot find my mnemonic word from bitcoin-qt.
Please let me know, how can I get my mnemonic words from bitcoin-qt.

BRs.
How most HD wallets work is that they take (or generate) a mnemonic, the mnemonic is used to generate the HD wallet master key, and the master key is used to generate the keys in the wallet. Bitcoin Core doesn't yet support the first part of that: it generates a random master key without using a mnemonic. So there is no mnemonic.

If you would like to get mnemonic phrase, you can generate one here 
or using electrum. 

Note: electrum mnemonic phrase is not supported everywhere.