My xbox one gift code for 20 euros does not work.

Smwutches is correct. Gift cards are region locked to the region they were purchased in or the region where that currency is accepted. For instance the code you purchased will be for Euros the UK does not accept Euros as payment.

There is no workaround

stored value

What happens if I buy a Microsoft gift card in Germany in euros and add it to my account, and then move to the United Kingdom? Does it convert from euros to British pounds?

Balances are not converted from one currency to another. You must spend the value of the currency in which it was purchased. You will have to buy another Microsoft gift card when you move to another country or region, because the balance you have in one
currency will not work in another marketplace.

game called Fartnite code with a gift card for 25 euros

Xbox uses local currency to represent points or currency to purchase

Even if It recognizes euro prepaid gift cards are region specific

Meaning even if you use a France gift card in a german account it won't work even if they both use euro

Most likely France will use francs anywsays

Thanks Darkplayer 2. I am looking to buy the Xbox gift card from an online vendor, like NewEgg, Amazon, etc... Which they occasionally have promo codes that will discount the Xbox gift card by 20%. So the $50 gift card would only cost me $40 after the
promo code.

This is good to know, b/c one review I read somewhere said the $50 card does not cover the everything and they could not split the transaction to cover the tax.