Can't sleep, is it ok to take melatonin supplements?

Yes: If you are otherwise healthy. Sugar in the diet sometimes interferes with sleep so take that into consideration.

Melatonin can be: Very effective for insomnia. Make sure you address sleep hygiene. That can make a big difference. Go to bed and get up at the same time each day. Make sure the bedroom is a comfortable temperature and that the room is dark & quiet. Avoid exercise, caffeine, alcohol in the evening/ night. No big meals before sleep. The bedroom should be for sex & sleep.