睡不着觉 means that you want to go to sleep,but you couldn't fall asleep.Maybe you are closing your eyes,but you are still awake. 不能睡觉 maens that you are not allowed to go to sleep.

睡不着觉 is mean you want to sleep on the bed, but you can not,maybe you drank too much coffee or tea or eat too much food. 不能睡觉 is mean you can not sleep,maybe you work on night shifts or you have to finish your report at night,so you can not sleep.

睡不着觉-can't go to sleep/be unable to sleep. I tried, but I still can't go to sleep last night. 不能睡觉-can't go to bed/can't retire. I have to work now, so I can't go to bed.

睡不着觉 means you've tried every way that helps you to sleep, but you still cannot fall asleep. 不能睡觉 means you mustn't sleep because of much work to do or waiting for a TV program , or somebody force you not to sleep.

ah！我明白了！謝謝您