If you're under 13 years old, only your parent can make your book public.
To do so:
1) Make sure that you have an "Independent" account type. To check this, click on your name at the top of StoryJumper, click "Settings", and scroll down to "Account Type". If it doesn't say "Independent", then you need to transition to an Independent Account.
2) After you complete step 1, your parent's account will be linked to your account. Ask your parent to go to their StoryJumper account, click on your book, click "Share", and then make your book public.