You want to make an event selection required in an event registration form. Perhaps you have multiple events within the same event registration form, and one of them is mandatory. You may want to require a registration fee. Perhaps you have multiple free event units on the part, but you want to require a flat fee for the total.
All events are optional in a NetCommunity event form.
The user must select 1 event price unit per event form. You may consider adding text to the page or to the event to note that one of them is required. It will be the participant's responsibility to choose the event.
If you have a scenario where there are multiple free events, but want to require a Flat Fee, add a price unit for the Flat fee and make a note on the page that it is required.