validate

projects.php

<?php
function projects_node_project_form_validate(&$form, FormStateInterface $form_state) {
//  $project = $form_state->getFormObject()->getEntity();
  $value = $form_state->getValue(['field_value', '0', 'value']);
  $sum = 0;
//  $status = $form_state->getValue(['field_status', '0', 'value']);

  foreach ($form_state->getValue('field_investors') as $index => $item)
    if (is_numeric($index)) {
      $sum += $item['subform']['field_contribution']['0']['value'] + $item['subform']['field_contribution_over']['0']['value'];
    }
  
  if ($value != $sum)
    $form_state->setErrorByName('field_value', 'Проверьте сумму, должна быть: ' . $sum);
  
}