Try ShortPoint nowModern 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.
