I have an American Amateur Extra Class licence (AC1DM) and I'm going to Portugal for a few days. Can I operate there with my licence? I'm not an American citizen.
After reading the various online sources, it looks like the answer to your question is no.
US amateurs can operate in CEPT countries, but a requirement is that they be US citizens.
Your user name suggests that you hold a licence in Argentina, but sadly Argentina is not a party to the CEPT agreements.
If your visit is only for a few days, it might not be worth it to try other options. It might be possible to get a reciprocal licence, and if you were resident in Portugal or travelling there often it would be worth considering. But if it’s just for a single trip it might not be feasible.