Cowboycork Hi
The Eurail Passes for Switzerland are valid on the direct buses Tirano - Lugano. You can find the validity map here:
https://www.interrail.eu/content/dam/_new-structure/doc/res/See_Switzerland_by_train_2024_Geltungsbereich.pdf
I think that you bought tickets for the train and not for the direct bus. To get a refund, call the Contact Center of the SBB with this number:
+41 51 225 78 56 (24/7, waiting times may occur)
Reservations for the direct buses are mandatory (16 CHF) and can be made here:
https://shop.rhb.ch/en/bernina-express
Choose “Tirano, Stazione” as departure, not “Tirano”.