Start from:
Communities > Forms > Single Step Donation

1.  Hover over the name of the form and click Website Info
2.  Click on the Donation Form Options link
3.  Ensure that, under the Recurring Gifts Options, either "Enable recurring gifts as an extra option" or "Enable recurring gifts as the only option" is selected
4.  Check the box for "Custom Schedule", found under Recurring Gifts - Payment Frequencies.
5.  Click Save
6.  Go back to the Website Info page for the form and click Publish This Event Now