Q: Why my Apple ID is not working for iTune Store

MacBook Pro 2012 Edition, OS X 10.11   8GB RAM, 256GB SSD

I cannot use my Apple ID to download any program from iTune Store (likes zinio,..., free program).

Because it failed to sign in iTune Store. (But it can sign in for Apple Store.)

I want to know why my Apple ID is not working for iTune Store?

Thanks so much