To preview your formatting with a page break in Atticus, please make sure to select 'Print' from the dropdown menu above the previewer.
The eBook previews in Atticus utilize a scrolling view rather than a paginated view, so the page break will not be visible in the preview unless you have selected the Print preview.
It's also worth noting that ePubs don't maintain formatting in the same way that print books do. The presence of page breaks in ePub format can vary depending on the device and layout used by the reader. When an ePub is viewed with a paginated layout, the page break should be applied, but it may not appear when the reader is using a device or layout with continuous scrolling.