Hopefully I can explain this well enough that someone can understand what I'm
asking.

I currently use an Excel spreadsheet to track daily attendance of employees.
This sheet is filled out by department leaders on a daily basis.

I have a column for time in, time out, and a leave code column (this column
is used to show the reason an employee is not at work).

Whether an employee works or not, these columns will be filled out (if
worked the time in/time out will be filled out and if not worked the leave
code column will be filled out).

I need to know how I could make the sheet show an error message if all these
cells are left blank by mistake (i.e. an employee doesn't work today,
therefore the time in/time out cell is left blank but their supervisor fails
to put in a leave code. I would like the error message to alert them to the
fact that the information in these cells are mandatory and that they have
left them blank).

Is this possible?

If I'm not making any sense, please let me know so I can clarify any
confusion.

Thanks in advance.