Warning: Illegal offset type (multiple) when creating new report from template (D8/Civi5.11)
Its a bug in CiviCRM and is not handled properly for D8, you can fix this by applying below patch

diff --git a/CRM/Report/Form/Instance.php b/CRM/Report/Form/Instance.php
index 8f00408a17..965949e21c 100644
--- a/CRM/Report/Form/Instance.php
+++ b/CRM/Report/Form/Instance.php
@@ -112,7 +112,13 @@ class CRM_Report_Form_Instance {
     if (function_exists('user_roles')) {
       $user_roles_array = user_roles();
       foreach ($user_roles_array as $key =&gt; $value) {
-        $user_roles[$value] = $value;
+        if (is_object($value) &amp;&amp; is_a($value, 'Drupal\user\Entity\Role')) {
+          $value = $value-&gt;label();
+        }
+        else {
+          $key = $value;
+        }
+        $user_roles[$key] = $value;
       }
       $grouprole = $form-&gt;addElement('advmultiselect',
         'grouprole',