Please try typing the birthday in without the forward slashes, for example, Jan 25th, 1970 would go in as 25011970. The TurboTax software will include the slashes. Make sure that you are putting the day first then the month and then the year.
If that doesn't do the trick, is it possible you're running TurboTax in a mobile browser? If so, try switching to a computer to enter your mother's birthday.