vsts-npm-auth can't get authentication token o

I'm attempting to use vsts-npm-auth to get the authentication token for our VSTS package repository. On my development machine I can run the commands

npm install -g vsts-npm-auth vsts-npm-auth -config path-to-my\.npmrc

and it succeeds in providing me with an authentication token. I'm now trying to recreate this as a build step on VSTS, so I create the powershell script auth-vsts.ps1

$npmrcFile = "$PSScriptRoot\path-to-my\.npmrc";
npm install -g vsts-npm-auth;
vsts-npm-auth -config $npmrcFile;

and add it as a powershell task. However, the task fails as follows

2017-05-30T09:37:41.1082686Z ##[section]Starting: auth-vsts
2017-05-30T09:37:41.1092712Z ==============================================================================
2017-05-30T09:37:41.1092712Z Task         : PowerShell
2017-05-30T09:37:41.1092712Z Description  : Run a PowerShell script
2017-05-30T09:37:41.1092712Z Version      : 1.2.3
2017-05-30T09:37:41.1092712Z Author       : Microsoft Corporation
2017-05-30T09:37:41.1092712Z Help         : [More Information](https://go.microsoft.com/fwlink/?LinkID=613736)
2017-05-30T09:37:41.1092712Z ==============================================================================
2017-05-30T09:37:41.1112679Z ##[command]. 'd:\a\1\s\auth-vsts.ps1' 
2017-05-30T09:37:47.3792461Z C:\NPM\Modules\vsts-npm-auth -> C:\NPM\Modules\node_modules\vsts-npm-auth\bin\vsts-npm-auth.exe
2017-05-30T09:37:47.3792461Z C:\NPM\Modules
2017-05-30T09:37:47.3802239Z `-- vsts-npm-auth@0.25.0 
2017-05-30T09:37:47.3802239Z 
2017-05-30T09:37:47.3802239Z 
2017-05-30T09:37:47.3802239Z vsts-npm-auth v0.25.0.0 
2017-05-30T09:37:47.3802239Z -----------------------
2017-05-30T09:37:47.3802239Z Creating npmrcFile. Path: D:\a\1\s\.npmrc
2017-05-30T09:37:47.3802239Z Getting new credentials for source:https://our-domain/_packaging/SharedLib/npm/registry/, scope:vso.packaging_write vso.drop_write
2017-05-30T09:37:49.8729702Z Caught exception: The prompt option is invalid because the process is not interactive.
2017-05-30T09:37:49.8729702Z Parameter name: PromptType
2017-05-30T09:37:49.8729702Z Caught exception: The prompt option is invalid because the process is not interactive.
2017-05-30T09:37:49.8729702Z Parameter name: PromptType
2017-05-30T09:37:49.8729702Z Couldn't get an authentication token for //our-domain/_packaging/SharedLib/npm/registry/:_authToken.
2017-05-30T09:37:50.1769711Z ##[error]Process completed with exit code 1.
2017-05-30T09:37:50.1809715Z ##[section]Finishing: auth-vsts

The error gives no indication as to why it can't obtain the credentials. Any ideas why this might be?

The error did indicate why it cannot obtain the credentials:

The prompt option is invalid because the process is not interactive.

This could be caused by the build agent does not run in interactive mode which make the credential dialog cannot be prompted. If you are using Hosted Build Agent, the build agent is run as service and there isn't any way to change to interactive mode.

However, the issue here is that if you want to use the feed in a build step, it does not make sense to prompt a credential dialog during the build process since the build step cannot enter the required credential automatically. Not sure if there is any specific requirement in your environment, but the general workflow should be uploading the .npmrc file generated in your local machine to the Source Control so that npm can use the auth token in the file to install/publish packages to VSTS Feed.