You need to get a Nokia music unlimited subscription on your phone. When you buy a new Lumia, it comes with 3-12 months of free downloads, depending on the model of the phone. After that period ends you can purchase vouchers to renew your download subscription.
the app voucher is not related to Nokia music. The unlimited Nokia music download is valid for a certain number of months from the date you activate it. How long has it been since you activated the account?
In that case you should be able to download music. In the Nokia mix radio app, go to settings, and check that you are signed in, and that your subscription is valid.
You can download music by going the the "store" section in the Nokia mix radio app.