signrawtransaction is not a function on bitcoin core V0.17.1
It looks like the new calls are not yet supported in btcClient.

https://github.com/ruimarinho/bitcoin-core/issues/77 refers generally to your issue.

v2.0.0 was released "a year ago" according to https://www.npmjs.com/package/bitcoin-core; v0.17 bitcoind, which made the change, four months ago.

If you're just tinkering, you might consider downgrading your bitcoind until the library catches up.