There are engineering works that weekend. You must take a replacement bus to Morbegno and then the train.

Rail Planner doesn’t show you all options though. Use trenitalia.com. For example:

• Bus Tirano - Morbegno 18:48 - 20:00
• R Morbegno - Milano Centrale 20:10 - 22:10

Option 2:

• Bus Tirano - Morbegno 19:38 - 20:50
• R Morbegno - Milano Centrale 21:00 - 22:40

Thanks for quick reply @thibcabe . I hope the pass covers this route for bus+train? Also if any idea how far the  bus stops from Tirano and Morbegno stations? 

Yes the pass covers everything.

Tirano bus station is south of the Italian train station, use the underpass. There will likely be signs.

I have no personal experience with Morbegno but looking at Google Maps there should be enough space in front of the train station.

@thibcabe could you help with below queries please.

      1. Do i need to book any seat for Bus and/or train? How can i make seat reservations for the bus as i would be travelling with family and don't want to miss that bus at 18:48 due to non-availability of seats

2. How can i add this journey (bus+train) to the pass? Manually?

You can’t book seats on trains or replacement buses on this regional route.

If the eurail app hasn’t been updated to show the correct train/bus times then yes, you can add these manually, use trenitalia website or app to get the correct times.

In addition: if you split the search (Tirano - Morbegno and then Morbegno - Milan) all connections seem to be there.

thanks again for quick response @Al_G  and @thibcabe