Added short prereq paragraph

Related to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30321

doc/ci/pipelines/schedules.md

@@ -21,6 +21,15 @@ Pipeline schedules can be used to also run [pipelines](index.md) at specific int
 In addition to using the GitLab UI, pipeline schedules can be maintained using the
 [Pipeline schedules API](../../api/pipeline_schedules.md).
 
+## Prerequisites
+
+In order for a scheduled pipeline to be created successfully:
+
+- The schedule owner must have [permissions](../../user/permissions.md) to merge into the target branch.
+- The pipeline configuration must be valid.
+
+Otherwise the pipeline is not created.
+
 ## Configuring pipeline schedules
 
 To schedule a pipeline for project: