I have a spreedsheet where the entries are Y, N, NA, N/A. or left blank and
the numbers of each totalled.

Even though the sheet is clearly marked what entries can be used, some users
of the sheet sometimes enter space Y instead of Y or N space A instead of
NA, zeroes etc. There are other various errors which result in incorrect
totals being shown.

How can I get a cell to show an error if none of the above are entered.
This would indicate to the user that they have made an error?

Can this be done?

Any help greatfuly appreciated.

Regards Bill