Why does it say: “Your apple ID has been disabled” when i want to download something ? But i can log-in to my account and no error.

It can happen for a number of reasons e.g. wrong password being entered a number of times, iTunes Support disabling it due to violations of the store's terms. And an account can just be disabled from downloading content, it doesn't necessarily mean that you can't just log into it.

Try resetting its password (and then logging out and back in everywhere) : If your Apple ID has been locked - Apple Support

If that doesn't fix it contact iTunes Support in the country where you are (and which is therefore the country on the account) and see if they can/will re-enable it : http://www.apple.com/support/itunes/ww

Your apple id have been disabled from the store.contact itunes support