There are two options for adding a new canvas to a page in your design. You can either add a canvas from another design, or you can add a new blank canvas.
How to Add a Blank Canvas
To add a new blank canvas to a page, first choose the page that you would like to add a new canvas to and navigate to the Page Tab. Then click on the three dots to the right of the page's name and choose to Add Blank Canvas.
This blank New Canvas will be added to the bottom of the canvas list in the left side Site Panel on your page.
Click and drag this canvas to place it where you want in your page.