Unable to upload .po translation file

When you try to upload a translation file (.po), it fails with this error : 

Warning: move_uploaded_file(): Unable to move '/tmp/xxxxxxxxx' to 'translations://xx-xx.po' in Drupal\Core\File\FileSystem->moveUploadedFile() (line XX of /mnt/www/html/[sitegroup]/docroot/core/lib/Drupal/Core/File/FileSystem.php)

Check that the "translations" directory exists at docroot/sites/[site]/files/translations

If it does not, please go to /en/admin/config/media/file-system and click on "Save", that will create the directory.