In the current version of the pipline-npm-plugin the generated npmrc file is not compatible with the version artifactory is using (npm v9).

The old syntax for the authentication of:
_auth=<username>:<password> (As base64)
does no longer work.
The new format required is
//<registry-url>:_auth=<username>:<password> (As base64)

as mentioned in this post by jfrog:
https://jfrog.com/knowledge-base/artifactory-changes-to-the-login-behavior-in-npm-v9/

As a suggestion I'd recommend that a templating syntax is added for how the _auth is constructed so both formats are possible.

Here additionally a screenshot of the received error message: