Translation:My mother cannot use the phone.
I've seen people with this problem a few times in this site. A piece of advice, forget the word "utilizar" except if you're going to use it in formal situations. Your translation with "usar" was perfectly correct, don't get to caught up with Duolingo's corrections and just enjoy the language :)
Here in America, people often use the word "utilize" instead of "use" in order to sound more intelligent, or more educated. I try diligently to never use "utilize" because it is stilted and officious.
Now I'm learning that both words exist in practically the same form in Spanish.
But, if I translate "utilizar" into a form of "to use", it seems to me that I am not just translating, but also editing the original Spanish text in accordance with my beliefs about effective writing. Therefore, I will translate forms of "utilizar" into forms of "utilize", and forms of "usar" into forms of "use". If I am asked to perform stylistic edits during translation, however, then I shall ruthlessly prune all "utilizars".
Why does Duolingo use 'utilizar' ? It can also mean to utilize, which is different from being able 'to use' something, which is 'usar'. In school we have phrases such as, 'a mi madre le gusta usar el ordenador.' (My mother likes to use the computer). Can someone explain why Duolingo uses utilizar?