Sharepoint - List Validation does not show Error when Conditions are not met

Couple of questions:

• What column data type are you using?
• What column validation formula are you using?
• Just to be sure, are you using the OOTB column validation feature and not some custom validation feature? If so, did you specify the user message property?

I am able to create a simple number column, and set a validation formula as follows

And, when I try to set an invalid value to the column, the specified error message shows up as expected.

EDIT:

It seems that you are using Date values in a choice column. You cannot use the '-' operator in that case, since the value will be considered a string. Can't you use a date column to store dates instead? Use another calculated column to check the difference in days, and use that calculated column value to set the choice.

When you create an item, the creation date is not set so it can't be used in condition.
You can use the current date to do what you want, or a combinaison of the current date and creation date to cover both creation and edit.

Do you use branding on the site ? The validation error elements could be affected by custom styles.

Search through the source of your page using developer tools or firebug and see if the message is there.