You need to make sure you select the artboard itself and then you can apply a time based trigger.
Check out Dani Beaumonts video: https://www.youtube.com/watch?v=Hsa_f-xdzHM
In this video Dani explains how to use trigger transitions on a timed delay using Timed Transitions.
If you selec the ARTBOARD name, rather than an object in the artboard, one of the Triggers that becomes available is Time. This is the way that you can automatically jump to another screen.