"One of the Parameter is invalid"

Someone pls. help, my formula is computing Aging Hours between two dates is DAYS({Date Completed},{Date Created}) * 24

The problem is “Date Completed” is null at the moment and my column shows red exclamation mark. I want to get rid of this error. Im trying to configure my formula to IF’s but still same error appeared.

IF({Date Completed},
MULTIPLY(DAYS({Date Completed},{Date Created}),24),
“not yet”

I still received error saying illegal formula.