This pipeline script appears to be attempting to assign the value for what should be a User variable in the script environment to an existing groovy script variable, resulting in this error: you tried to assign a value to the class 'hudson.model.User' @ line 11, column 5.

Perhaps this is expected behavior? I would assume these variables should be in the context of env. Maybe this is a special case?

pipeline {
  agent any
  stages {
    stage('Build') {
      steps {
        sh('mvn package')
      }
    }
  }
  environment {
    User = 'asdf'
  }
}