I’m in UK at the moment , and my account is EU region. Bought “Blizzard balance to buy 60 days game time” card at our local Tesco’s but when I enter the code it says “Sorry can’t claim this code in your country”. The price tag printed on the card is in pounds so it’s realy for EU region 
They can’t deal with account and payment issues over the forums due to privacy and security concerns.
Nor can they assist with things bought from third party retailers, which is what Tesco’s technically is.
My advice would be to return it to Tescos, but they are likely to be highly resistant to that, as theres no way they can validate that you are not trying to return a used card.
Your only real other option is to take out a ticket and wait to speak to a GM.
I think i know the answer 
most likely you tried to enter code on US try to use this link and enter code here:
https://eu.battle.net/shop/en/checkout/key-claim
just be sure it is EU in www address, not US.
Adding to what Decayin said, if you’re definitely on the right page, is your account registered to an address in the UK? That’s the only way you can use a card in £. If you’re from elsewhere in the EU, you’d need a card in Euros.
If all that checks out, contact a GM to see if they can work out why the site doesn’t like the card.
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Hello everone 
Thank you for jumping in and helping.
Contacting us is the way to go in this case!
We have a guide for such cases as well, that is why it is always best to first check our knowledge base.
Since you say you are in the UK at the moment, that probably means your registered country is not the UK so therefore it’s the wrong currency.