Cannot add attachment to existing icalendar event

I have seen other people post this question with no usable answer. I have an existing calendar event in the iPhone calendar app. I am using an iCloud calendar. When I create a new event, there is an option to add an attachment. However, if I have an existing event and want to edit it and add an attachment, the “Add Attachment” button is not visible. It simply is not there. I am running IOS 15.5