Per this module, I am trying to create a validation rule that fires if IsClosed is not selected and CloseDate is yesterday or earlier.  I've written this formula:
and(
not(IsClosed),
today() >= CloseDate
)

The error I'm getting on trailhead just says that this isn't working properly and I should double-check it.  Anyone have any ideas to point me in the right direction?

Thank you in advance!

Your company sells cylindrical hyperbaric chambers. You need a formula field that calculates the volume of a cylinder for you, rounded to the nearest whole number, given its radius and height. Use the existing mathematical formula for the volume of a cylinder, V = πr2h, where r is the radius of the cylinder, h is the height, and π is the constant Pi. Note: Although this formula field might best be created on a custom object, for simplicity, we’ll create this formula on the Opportunity object.
>Create 2 custom fields of type Number on the Opportunity object: ‘radius’ with a resulting API name of ‘radius__c’ and ‘height’ with a resulting API name of ‘height__c’.
>The formula should be named ‘Cylinder Volume’, with the resulting API name ‘Cylinder_Volume__c‘ and should be created on the Opportunity object.
>The formula should reference the custom fields ‘radius__c‘ and ‘height__c‘.
>The formula should use 3.14159 as an approximation of Pi.

Heres the challenge that im stuck on. i have created the 2 custom fields. What im stuck on is where do we create the formula. Do we put it in the custom fields or is there another place u insert it?

thanks if anyone can help