Modern ShortPoint Theme Builder is only for Modern SharePoint pages so when you open Modern Theme Builder on classic page, you will get two options:

Option 1: Switch to Classic theme builder


Clicking on Switch to Classic button, ask you to discard any changes (if there are any) you did in Modern Theme Builder and re-activate Classic ShortPoint Theme Builder.

If you don't want discard your changes, you can publish your Modern ShortPoint Theme Builder changes and open Classic Theme Builder from ShortPoint Dashboard.

Option 2: Navigate to modern page

On the preview pane right side of your page, redirect to Classic Page.