Confusion about covid entry requirements for Portugal

Airlines almost always base their board / don't board decisions on what's shown in the IATA Timatic database. You can access that database here.

IATA's covid-focused database is here. Click on the country outline for Portugal, and read what IATA thinks the requirements are.

Two notes. First, you'll find that entry requirements may be affected by your country of citizenship, where you're flying from, where you've been in the past month or two, and sometimes even what other visas or residencies you might have.

Second, pre-flight testing these days is a high-risk issue. That is, if a test is required and you don't have a test, you won't fly. Not flying with foul up both your schedule and your arrangements and your pocketbook, because buying a last-minute replacement air ticket is always much more expensive than buying well in advance.

If answers differ, you may reasonably conclude that the most prudent course of action is to obtain a test.

Welcome to unholy mess that is travelling during Covid.

A few things to consider

1. You can check IATA requirements for your specific details yourself here: https://www.iatatravelcentre.com/passport-visa-health-travel-document-requirements.htm
2. If United has accepted your documentation and your status is green on the Travel Ready Center, it's very likely that you can just check in online and they will allow you to board without any further checks.
3. I flew on United to Germany two days ago with similar requirements: United accepted my vaccination records (including the CDC card) on the second attempt (no idea what was wrong in the first submission). There was a second check in London for my flight into Germany but I have an EU certificate that worked there. I strongly recommend bringing printed boarding passes for all flights (if you can get them at check in)
4. A good resource for requirements in Europe is https://reopen.europa.eu
5. It's unclear if you still need to fill out a passenger locator form (PLF). According to https://reopen.europa.eu you need one. According to https://www.visitportugal.com/en/content/covid-19-measures-implemented-portugal#:~:text=Children%20under%2012%20do%20not,at%20the%20time%20of%20boarding. the requirement has been revoked
6. PLF for Portugal is a royal pain. We went there last in late Feb: you need a seat number, so you need to check in first. However you can't check in without a PLF so it's a classic catch-22. Once you figured this all out, they send you a confirmation. It's password protected and they don't tell you what the password is (turns out it's your passport number). As a result of this chaos, our flight left over an hour late because it took forever for passengers to sort out their paperwork.
7. Acceptance of the US CDC card in Europe is spotty. I haven't tried the Smart Heath Card yet. On the other hand, once you are in Portugal, you should not need one anymore.
8. You will need a test for the return to the US. We got one in a public test site (tent) which was quick and relatively inexpensive. I uploaded the test for online check in and it was rejected. However it got accepted by the check in agent. I don't know why.
9. There are multiple testing locations in Lisbon airport as well. We asked one of test centers whether their test is good for the US. The answer was "We don't know".

What do I do here?

The safest option by far is to get a test. You probably don't need one, but its very difficult to predict this with any certainty and doing a last minute scramble at the airport is nerve wrecking and expensive.