Hi john john It is a default behavior that attachments cannot be added in Modern Event Web Parts and not because some settings are disabled.
Currently, you can attach files to event list items in classic experience by following below steps:
1.Create an event list in classic experience
click Site Contents > click + New > click App > search for Event > add the Calendar.
2.Add item and attach file.
open the created event list > click +add
3.Add event.
click edit in the upper right corner > click Edit web part > choose the created event list earlier
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