Computers aren't very smart. Sure, they can do a lot of math or help you search the Internet. But, if you asked a computer to vacuum the house for you, could it do it? If you asked a computer to draw a picture of a bird for you, would it? A computer would have no idea about what you're saying.
Computers are bad at understanding things. If you don't give them exact instructions, they become confused and make mistakes. Telling a computer what you want it to do is sometimes hard because you have to explain things very carefully.