diff -u b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
--- b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
@@ -267,7 +267,7 @@
           $this->errors[$element['#name']] = $msg . ' ' . $e->getMessage();
         }
       }
-      else if (!file_exists($source) || (!is_dir($source)) || (!is_readable($source))) {
+      elseif (!file_exists($source) || (!is_dir($source)) || (!is_readable($source))) {
         $this->errors[$element['#name']] = $msg;
       }
     }