While using sidekiq-scheduler if your schedule file is named as sidekiq.yml make sure to add the schedule config entry.
i.e :scheduler: -> :schedule .
For eg:

:scheduler:
  :schedule:
    fetch_user_info_from_slack:
      cron: '0 6 * * * Asia/Kolkata'
      class: FetchSlackUserInfoJob
      queue: 'default'
      description: 'This job fetches user info from slack and updates the database'