This is IT-noted for incomplete or outdated or even false info-that could also change every minute. Esp in what they think is main summer the service is very often curtailed-at last minute-to suit lower demand.

Maybe there are works on that line/stazione just on that day. May be they have not even decided. Anyway its way too early now for me-often there is simply from a 3 month advance timespan.

Also quite possible this train starts Centr and then also stops PG.

AS we have told here so many dozens of time: app is often outdated-use bahn.de or always best the local railway=trenitalia for best=hopefully most accurate info.

Always use the website of the railways of the country you are traveling in/to.
I've done it for you now, and this train does depart from Porta Garibaldi, and is already bookable.