Can I leave my suitcase at the Rossio Metro Station in Lisbon

If I am reading this right, you arrive in Lisbon 6 hours before the officially-stated check-in time at your hotel? Is there a reason you can't leave your bag at the hotel, if you cannot get your room that early?

Perhaps they rented an Air BnB. and this is often a problem with them. Hotels will hold luggage until check in time.

It is an airbnb

In addition to self-operated luggage lockers in train stations, there are many private stores that also offer short term luggage storage. Quite likely, your bnb may recommend one nearby. Several websites for Rossio station say there are several options within the station.

I don't see a problem. We did that ourselves a couple of years ago. If the Rossio lockers are full, there's one really close at Praça Figueira which is what we ended up using. No worries with finding the lockers after wandering around with jetlag. The steps to go to the parking area are a bit behind the statue. Find the statue, find where you left your luggage.
https://www.citylockers.pt/lisbon-rossio/