‘checkout scm’ cannot be run from Pipeline script

[modestMouse1]

I'm trying to run a parameterized Pipeline script as follows:

def doGitCheckout(github_url, branch_tag_identifier, github_key) {
  checkout([ 
    $class: 'GitSCM',
    branches: [[name: branch_tag_identifier]], 
    doGenerateSubmoduleConfigurations: false, 
    extensions: [
      [
        $class: 'CloneOption',
        noTags: false,
        shallow: false,
        depth: 0,
        reference: ''
      ],
      [ 
        $class: 'SubmoduleOption', 
        depth: 1, 
        disableSubmodules: false, 
        parentCredentials: true, 
        recursiveSubmodules: true, 
        reference: '', 
        shallow: true, 
        trackingSubmodules: false
      ],
      //[$class: 'GitLFSPull']
    ], 
    submoduleCfg: [], 
    userRemoteConfigs: [[ credentialsId: github_key, url: github_url]]
  ]);
}
/****  PIPELINE ****/
pipeline{
    agent any
    stages{
        stage("Clone & Build") {
            steps{
                print( "repository URL: " + scm.userRemoteConfigs[0].url );
                doGitCheckout(
                    scm.userRemoteConfigs[0].url,
                    'refs/'+params.branchTagIdentifier,
                    'github-key'
                );
            }
        }
    }
    post{
        always{
            script{
                    emailext(
                        body: '''${SCRIPT, template="my-email.template"}''',
                        mimeType: 'text/html',
                        subject: "[Jenkins] ${env.JOB_NAME} [${env.BUILD_NUMBER}] - ${currentBuild.currentResult}",
                        to: '$DEFAULT_RECIPIENTS'
                    );
                cleanWs(deleteDirs: true, disableDeferredWipeout: true);
            }
        }
    }
}

When run from within Jenkins, the script runs fine, when run from VS Code using Jenkins Jack, I get the following error:

ERROR: ‘checkout scm’ is only available when using “Multibranch Pipeline” or “Pipeline script from SCM”
Finished: FAILURE

I'm using GitHub plugin v 1.34.3.