  pipeline
  {
      agent none

     options
     {
        ansiColor('xterm')
        buildDiscarder logRotator(artifactDaysToKeepStr: '30', artifactNumToKeepStr: '10', daysToKeepStr: '30', numToKeepStr: '10')
        timestamps()
     }

     stages
     {
        stage('Install and Validate')
        {
          parallel
          {
            stage('Ubuntu 20.04')
            {
              agent
              {
                label 'linux'
              }
              steps
              {
                sh 'echo test'
              }
            }
          }
          stage('Ubuntu 18.04')
          {
            agent
            {
              label 'linux'
            }
            steps
            {
              sh 'echo test'
            }
          }
        }
     }
  }
}

pipeline
{
    agent none

   options
   {
      ansiColor('xterm')
      buildDiscarder logRotator(artifactDaysToKeepStr: '30', artifactNumToKeepStr: '10', daysToKeepStr: '30', numToKeepStr: '10')
      timestamps()
   }

   stages
   {
      stage('Install and Validate')
      {
        parallel
        {
          stage('Ubuntu 20.04')
          {
            agent
            {
              label 'linux'
            }
            steps
            {
              sh 'echo test'
            }
          }
        }
        stage('Ubuntu 18.04')
        {
          agent
          {
            label 'linux'
          }
          steps
          {
            sh 'echo test'
          }
        }
      }
   }
}

Fix syntax error. Add stage.
Fix syntax error. Add stage.
