Managed WordPress keeps 30 days' worth of backups for you automatically — there's nothing you need to do to enable them.
Note: The Worker Plugin for Pro Accounts is required in order for backups to run. (Pro accounts are available in the U.S.A. and Canada.)
If you want to use one of your backups to restore your site, see one of these articles:
Note: Backups are only available for active Managed WordPress plans. If the plan is removed due to failed billing, you will need to restore from local backups or contact support for additional options.