Hello Nguyen
I went into the lesson and checked all of the trinket apps and they look like they’re functioning. In order to have output appear in the right pane of the app, you must either click on the “Run Interactively” button that looks like this, 
or the “Run” button that looks like this: 
.
If you still have problems, give us some more details about what you see, which trinket has the problem, and show us some screenshots as well.
I hope this has been helpful!
Laz