Fatal error Class ‘DateTimeImmutable’

After inserting date custom field in a test page, I cannot edit nearly anything: Questions and answers, formulas, scores are no more editable (you can see the label, but you cannot open their content).

Users can do the test, but instead of the result page, they get this error:

“Fatal error: Class ‘DateTimeImmutable’ not found in /htdocs/public/www/wp/wp-content/plugins/wp-testing__fields/src/Model/FieldType/Date.php on line 35”

If you disable “Wp-testing — Fields” plugin, the editing test page is shown correctly again.

How can I solve or just remove the data field?

Thank you