Generally, setting a cell to flash is very annoying to the users. It
can be done, but once they get the point, it's distracting to have the
cell constantly flashing.
If you just want to be sure they do something in particular, like put a
value in Cell D5, try setting up conditional formatting in that cell so
that it has a bright fill color when blank.
You could also set up a different cell to be a reminder:
A1: =IF(ISBLANK(D1),"Cell D5 must have a value!","")
Then set up that cell with conditional formatting based on Cell D5.
Alternatively, you could set up a macro to run upon workbook opening
that displays a message box with the reminder.
Would any of those ideas work for you?
Ron