Stating the main conflict of this book is a bit difficult. Schmidt intricately weaves a few major conflicts throughout Trouble, and I believe different readers will see a different main conflict.
One central conflict is definitely a man vs. nature conflict. Henry and the other three climbers struggle to climb Katahdin. As with all mountain climbing, it's dangerous. The group is risking their lives, so I would be remiss to think this part of the story is not a central conflict.
There is conflict within the group, too. Henry wants to do the climb alone, but his best friend won't let him. Chay Chouan also wants to climb with the group. He is the guy who was supposedly driving the car that killed Henry's brother. Lastly, Henry's little sister wants to join the group. If you've grown up with younger siblings, you know there's conflict when they always want to "tag along."
Internally, Henry struggles to come to terms with the real person his brother was. Henry idolized his older brother Franklin, but Henry learns Franklin wasn't all that wonderful to other people.
There are other conflicts present throughout the book too, and a central theme of the book focuses on the book's title—Trouble. Trouble and conflict can't be avoided. The key to living well is appropriately managing and dealing with trouble.