Two possibilities come to mind:
First: If the settings for who can execute analysis (globally or on the project in question) are not set to “Anyone”, you need to provide credentials in your command. Check out the sonar.login and sonar.password analysis properties and documentation specific to SonarQube Scanner for MSBuild. Some more information about Authorization and permissions are available here.
The second, more of a “gotcha” possibility is that if the project you specified doesn’t already exist on the SonarQube server, the scanner will attempt to create the project with the key supplied.
If the user running the scan doesn’t have permission to create projects (or no login information is specified, see above), I believe it will fail with an insufficient privileges error. If you want to keep tight control on what projects are created, and make sure new ones don’t sprout out of a mistyped argument, I would create the project beforehand and seperate those permisisons from the user you use in your scan.
Hope this helps,