Hi, why do I keep gettnig this error in the challenge even though it clearly exists: - What am I missing...thanks

Dear All,
  I need to add a fillter on the date field, it must be less than or equals to today, that means the today date must be when the report is run.
In the calendar, I select Today, but that date is static, and would be yesterday when I run it tomorrow.
So it is not good using the Today in the calendar. PLease advice. Thanks a  lot

I am trying to finish the challenge in first chapter of formulas and validation in beginner trail(developer) to calculate the number of days between today and last activity date on a custom object.when I click on check challenge! I always get error message saying formula is incorrect.

here is the formula I did. TODAY()- LastActivityDate.
can anyone help?