Deriving hex private key from a HD wallet
The mnemonic represents a BIP32 extended private key: xprv9s21ZrQH143K4BbpSvRBDykoXLhwtznbp7qAfKVLw1QdJtJsmgS5rZJfSgZNtgk7e39uRNiYHbcRQnYvruqfftfG6WFmuiCspehEz1VS3YM

The function you are calling accepts a regular private key.

You can derive child private keys from the extended private key and use these to generate addresses.

It it be possible to derive the corresponding private key of either the children of this extended private key or the corresponding private key itself using the toWIF method in the bip32 JS library. See https://github.com/bitcoinjs/bip32/blob/master/index.js

You can see some examples of the use of BIP32 here:
https://github.com/bitcoinjs/bitcoinjs-lib/blob/a6f2cc2924df170537f0b588254178f5445d7dc8/test/integration/bip32.js