Maybe you're not old enough. Since this is anonymous, I can't tell, but kids - especially young girls - go online and can get into all sorts of trouble. If you're under 13, you can't join some social networks. Kass is right: She's probably just worried.
This really depends on your age. With most sites, you need to be 13+. I am a teen and I am not allowed Facebook. And honestly, most of the things on wattpad these days is rubbish. It's truly disgusting, not how it used to be where aspiring writers came together and created amazing stories! Now it's just dirty fan-fiction written by 12 years olds about 1D and 5-SOS. It also depends on how responsible you are too. Talk to your parents about it maturely if you are legal age (13+) and explain that you will use it well etc