Automatic renewal is legal, but Netflix needs to have warned you clearly in advance about the charge. According to consumer law, they have to inform you when it's expiring and ask for explicit confirmation before renewing. If they didn't do that properly, you have the right to complain and reverse the charge.
Start by contacting Netflix directly - log into your account, go to "Help" or "Contact Us" and explain what happened. Ask for a refund of that wrongful charge. Save screenshots of everything, especially the date you saw the charge and any communication from them. If they respond badly or refuse without valid reason, you have the right to file a complaint with your bank and request a chargeback. Often the bank can reverse it in a few days if you show that you didn't authorize it.
If Netflix doesn't resolve it in reasonable time, you can also file a complaint with the DGECC (Direção-Geral da Economia e Competitividade) or use online dispute resolution platforms - there's a European one (SOLV.EU) that accepts complaints about digital services. But honestly, try to resolve it directly with them first, they usually get refunds sorted quickly when they see the person knows their rights.