Webpagetest local instance.

https://github.com/marcelduran/webpagetest-api/blob/master/README.md#command-line using the above URL as a reference, installed npm package locally and while executing that I see this error. what can be done to resolve this?
:~ user$ webpagetest test https://lowes.com
below is the error.
{
“error”: {
“code”: “UNABLE_TO_GET_ISSUER_CERT_LOCALLY”
}