The payment frequency tab sets how often the payment will occur, in most cases it will be set to "Once off"

Once off - the payment only occurs once ( most common ).
Series - a second or third payment can be set at a later date on the "Event Prices" tab.   (Must have at least 1 Event Category)

Once an event is setup with Event Categories and pricing details and thus "Finished", the options are greyed out and cannot be changed.

Register Now can change it for you, however we will also advise what other changes need to be adjusted with the event setup when the changes are made.