why i can't download free skype app on my i pad

If your restrictions are enabled, then that explains why you cannot download apps. You are restricted from doing so.

Otherwise, your Appke ID might have a credit card or identity or related issue.

You haven't told us any specific symptoms. But if restrictions are in place, that is likely the problem.

Either your app downloads are restricted (check Settings > General > Restrictions) or there is some issue with your Apple ID account.