I'm unable to set the north angle for my model

When you first create a model, your model will not contain any geographic orientation or coordinates. You can always use one of the three axes as a placeholder for a cardinal direction. For example:

• Green is North.
• Red is East.
• Blue is Up.

If you want to orient your model using real-world coordinates you can use Add Location. With Add Location you can geo-locate your model to a precise location. SketchUp for Desktop users can also use an extension like Solar North.