how to create a username for instagram
I had tried to use my twitter username and I wouldn't let me
The Twitter username that you already tried out for your new Instagram account is most likely already taken. Try to think of a unique name that you can remember and which will also be appealing to you. Check out the rest of the directions for starting out on Instagram at http://help.instagram.com/customer/portal/articles/95811-getting-started-on-instagram.
Probably because it was taken. Make up another username.