now it works

stats/Jenkinsfile

@@ -1,9 +1,11 @@
  node {
     checkout scm
+    stage("File stats") {
       docker.image('jvncziii/ium:latest').withRun('') { c->
-        copyArtifacts filter:'train.csv', fingerprintArtifacts: true, projectName: 'z-s495716-create-dataset', selector: buildParameter('BUILD_SELECTOR')
+          copyArtifacts(projectName: 'z-s495716-create-dataset', selector: lastSuccessful(), filter: 'train.csv')
           sh 'chmod 777 stats/data_stats.sh'
           sh './stats/data_stats.sh'
           archiveArtifacts artifacts: 'train_lines.txt', followSymlinks: false
         }
       }
+  }

stats/data_stats.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 figlet "Counting dataset lines"
-wc -l cutoff_train.csv > train_lines.txt 
+wc -l train.csv > train_lines.txt