[Swift] Get private key in base58-encoding

[AlexDarsky]

Hello,
I'm using walletCore(swift) to signrawtransactionwithkey for siging inputs for raw transaction.But the response always is "Invalid private key".

How do I get the base58 encode string is :
{
let wallet = HDWallet(mnemonic: mnemonic, passphrase: "")
let derivationPath = CoinType.bitcoin.derivationPath()
let privateKey = wallet.getKey(coin: .bitcoin, derivationPath: derivationPath)
let privateKey58 = Base58.encodeNoCheck(data: privateKey.data)
}

Finally, the server replied to my error "Invalid private key"Is it that the steps I used are wrong?
Is there any error in my steps?