Required box not appearing, Event Registration
you will get this error due to following reason -


First if your Event registration is creating an New contact and if you have required field in your contact creation you will get this error message 
OR If you have some custom field created against Event and if those custom field are required fields then you will be getting these error message 
OR If you have any payment processor involved in your Event registration and if the address field are not field properly or card details are missing still you will get these error message 


So just make sure you find the required field first and try to set some default value or include the fields in the event registration 

If nothing helps please give some example - thanks