"Estar seguro/a" means "to be sure", "ser seguro" is to be safe. I was just used to saying "no estoy segura" during Spanish conversations so I got that part correct, but I answered "You can't be TOO sure" but it was wrong; should be "SO sure".
The first time I wasn't paying attention and put "I cannot be very sure" so DL corrected me to "You cannot be very sure"... The next time around, I put "You cannot be very sure" and it marked me wrong again correcting it to "You cannot be so sure". Wtf!!!!