Hello Elisabeth,
I am not clear about your situation. What kind of card is this? If it is a gift card you cannot redeem a card from a different country than that to which your store is set. The same applies to using a credit card from one country in another country's store.
If you really want to change country as in you have actually moved to live in a different country, see the directions in this link:
Please read this link. It describes how to change your country setting and the requirements and results of doing this:
Click on the link and read how to "Change your Apple ID country or region" - https://support.apple.com/HT201389
Note, I believe if you have unspent balance it will actually say that to you. In the past most people who have seen "storefront" have been trying to redeem gift cards from another country. If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support