I keep getting: Challenge Not yet complete... here's what's wrong:
The validation rule does not reference the 'IsClosed' and 'CloseDate' fields correctly. Take a look at the requirements again.

When I test the formula it seems to work as the requirements indicate yet it does not pass the validation from trailhead. Here is the formula: AND( NOT(IsClosed) ,  CloseDate < TODAY() )
It is placed on the CloseDate field.