Add message about existing bug to docs

Add workaround to bug
(https://gitlab.com/gitlab-org/gitlab/issues/208676) in docs of the
self-monitoring project.

doc/administration/monitoring/gitlab_self_monitoring_project/index.md

@@ -62,3 +62,29 @@ You can add custom metrics in the self monitoring project by:
 
 1. [Duplicating](../../../user/project/integrations/prometheus.md#duplicating-a-gitlab-defined-dashboard) the default dashboard.
 1. [Editing](../../../user/project/integrations/prometheus.md#view-and-edit-the-source-file-of-a-custom-dashboard) the newly created dashboard file and configuring it with [dashboard YAML properties](../../../user/project/integrations/prometheus.md#dashboard-yaml-properties).
+
+## Troubleshooting
+
+### Getting error message in logs: `Could not create instance administrators group. Errors: ["You don’t have permission to create groups."]`
+
+There is [a bug](https://gitlab.com/gitlab-org/gitlab/issues/208676) which causes
+project creation to fail with the following error (which appears in the log file)
+when the first admin user is an
+[external user](../../../user/permissions.md#external-users-core-only):
+
+```text
+Could not create instance administrators group. Errors: ["You don’t have permission to create groups."]
+```
+
+Run the following in a Rails console to check if the first admin user is an external user:
+
+```ruby
+User.admins.active.first.external?
+```
+
+If this returns true, the first admin user is an external user.
+
+If you face this issue, you can temporarily
+[make the admin user a non-external user](../../../user/permissions.md#external-users-core-only)
+and then try to create the project.
+Once the project is created, the admin user can be changed back to an external user.