In this helm installation link, https://zenko.readthedocs.io/en/latest/installation/install/install_helm.html, the download helm curl command (curl -LO "https://get.helm.sh/helm-v{{version-number}}-linux-amd64.tar.gz") does not fetch the correct tar ball.
That command fetches a file of type XML 1.0 document, UTF-8 Unicode (with BOM) text and this is untar-able.
The correct way to fetch the tarball is to go to helm's github release pages and download the tarball directly from there.
The correct tarball file type is gzip compressed data, last modified: Wed Jun 10 19:04:20 2020, from Unix
You can check by running file <fileName in terminal.