error

org.jenkinsci.plugins.workflow.steps.MissingContextVariableException: 
Required context class hudson.model.Node is missing

Perhaps you forgot to surround the code with a step that provides this, 
such as: node

Jenkinsfile

pipeline {

    agent none

    environment {
        JAVA_HOME = "${tool 'jdk11'}"
        MAVEN_HOME = "${tool 'Maven'}"
    }



    parameters {
        choice(choices: mont_blanc_envs("gke-preprod env: prod"), description: 'Env to run.', name: 'MONT_BLANC_DEPLOY_NAME')
//         string(defaultValue: '-Dcucumber.options="--plugin json:target/file-payment-cucumber-json-report.json --tags \'@payment and not (@ignore or @unstable or @bug)\'"
        string(defaultValue: '', description: 'Tag for tests to be executed', name: 'runTag', trim: false)
        string(defaultValue: '(@ignore or @unstable or @bug)', description: 'Tag for tests to be ignored', name: 'ignoreTag1', trim: false)
//         booleanParam(defaultValue: false, description: 'Do you want to run SonarQube analysis?', name: 'runSonar')
    }


    stages {

        stage('Purchase Expense') {
            agent { label "kubernetes && ${k8s_env.name}" }
            steps {
                script {

                    def auto_test = false
                    if (branch_name == "develop") {
                        auto_test = true
                    }

                    mont_blanc_deploy(auto: auto_test, this.&purchaseExpenseTests)

                }
            }
        }
    }
}