Dreaming of not being able to write an exam is a warning sign as per dream meaning and interpretation. The dream means you will face problems during an important appointment. You need might face trouble with vehicles. Dreams of not being able to write an exam also means sudden health issues or personal problems resulting in giving up on something very important.
Dream of not being able to write an exam and you see other people means postponement of something and this will result in lot of adjustments and troubles. The dream also means you might show carelessness in an important matter.
Dreaming of not being able to write and exam and you are alone in the dream means struggle to prove that you are not wrong. It also means people not showing kindness to you.
Please note that such dreams happen when you are intensely preparing for an exam. It does not have any meaning then as it is a continuation of the powerful daytime activities. A dream to have meaning it should happen naturally.