Hey Michael,
By referring to the attached screenshots provided, is that the button which you mentioned is not working? The second screenshot is from my local site and you can see the 'Attend this Event' button displaying correctly.
If this is the issue you are referring to, then it happens both in desktop and mobile when the user have not attend the event.
I suspect this issue is caused by template override. Maybe you can consult your template provider and see if the issue can be fixed.
If you are referring to another button, perhaps you can make a screenshot and show us where that button is.