The payment frequency is set as read only when:

• The "Payment Frequency" is set on the URL Token or passed in through a URL Parameter
• Payment Options are used (e.g. memberships) and at least one Payment Option does NOT have a frequency of One-off.
• URL Parameter or URL Token "paymentOptionIdReadOnly" is set to true

If the payment frequency being read only is not desired, you may wish to use the "Default Frequency" URL Parameter or URL Token value.