Am I doing something wrong?

It is probably because the planes used on that route are not configured with Premium Economy seats. Thye may only have economy, economy plus, and business. It seems to me I saw the same issue last month when I was considering my options if I had to re-book on a Covid Tested flight. (I booked PDX-SFO-FRA-BLQ back in December.)

You can check for yourself by using Seat Guru (https://www.seatguru.com/). Enter the airline (United), date, and flight number, and Seat Guru will show the cabin layout (along with individual seat ratings, like if there is an under seat obstruction, missing window, close to lavatories, catering, etc.).

Is it possible you are too early ? Some.airlines don't allow booking more than six months away. Just a thought.

You might try booking in economy and before you finalize the purchase, they may offer the upgrade at that point. I seem to remember something like this happening to me as well.

Alan is right, the United 767 has no premium economy.

According to United they fly 3 versions of the 767-300

https://www.united.com/ual/en/us/fly/travel/inflight/aircraft/767-300er.html

The seating charts in the link indicate that the version with premium economy also has the most business class seats and fewer economy seats. Given that business travel has been limited, it’s no surprise to me that United doesn’t plan to use this specific airplane layout. Of course if business travel bounces back they could decide to switch plane types.

The plane has 'economy plus'.

After selecting an 'economy' fare you get an opportunity to select from 2 'bundles' for. an econ plus seat, at an added cost.

Just keeping going in the process of selecting flights. It happens before any payment info is required

Allan (and Rocket) nailed it: "It is probably because the planes used on that route are not configured with Premium Economy seats. They may only have economy, economy plus, and business."

Most (huge generalization) airlines have just one level of Economy Plus or Premium Economy (they all use different names) but United actually has 2 different versions of "up leveled" Economy. One called Economy Plus (usually just more leg room with standard Economy seats) and a better version called Premium Economy (more leg room, wider seats, and some other perks. Which one you get depends on the aircraft being used on that particular route.

Thank you all so much!!! I am not an experienced flyer -- maybe flying only once every 2-3 years, therefore not very savvy about all of this. Your knowledgeable response is MUCH appreciated! Thanks again, Rose

I went back to Seat Guru last night to double-check my hypothesis. In October they are still running a 777-300ER on UA40 which has Premium Economy seating. But looking at February they've switched out the 777 for a 767-300. That just reflects the reduced winter demand.

I'm not certain of your exact travel dates. It looks like Alitalia is flying a 777-200ER out of JFK in February with Premium Economy, if that works for you. It's a code share with Delta and Air France.

if you use Google Flights you have more ways to search more options. You might even have to leave out of JFK to get what you want, although I believe United has moved everything to EWR. If you're trying to use miles, also look into other members of the Star Alliance where apparently you'll also often get better redemptions. The Points Guy is your friend here.

As an aside, apparently through some quirk of international aviation law you can fly Emirates from NYC to Milan if that works, and they are supposed to be one of the best airlines in the world.

We are flying to Rome in February and we booked with Delta's website and have seats on an Alitalia flight and premium economy. Of course you will have to fly out of JFK rather than Newark. Is that a problem?