If your Khan Academy account isn't automatically enabling you to participate in our Support Community, that is because we don't have an email address associated with your current Khan Academy account. Our support software (Zendesk) uses email addresses in order to keep track of accounts that post in the Support Community, so once you add an email to your Khan Academy account, you should be good to go. You can add an email to your account in your Settings, under Linked Accounts.
If a user is under 13, they will be restricted from leaving public comments in the Support Community in order to protect the child's privacy. More information on the differences between child and adult accounts can be found in this article. Everyone can reach our support team privately for help by filing a new ticket describing their situation.