Thanks for reaching out to the HubSpot Community! Glad to help here.
Are you able to provide more information on your end goal? I'm finding it's only possible sync over themes and template assets into a sandbox account as described in this article. This is an enterprise account feature.The pages cannot be copied or cloned to a sandbox. You would need to create a new page from the templates synced to your sandbox. We don't typically recommend using a standard sandbox account for a website redesign, as the redesigned templates and pages cannot be transferred to the main account.
Others have also come up with custom API solutions to transfer page content like found in this idea post here. You would need to work with a developer on this potential solution.
Another option, depending on your needs, would be to use the content staging tool. With the content staging tool, you can redesign and relaunch your website and landing pages in a staging environment. You can redesign and replace existing HubSpot-hosted pages, or create new pages from scratch. This is a CMS professional feature.
At this time, it's not possible to use content staging or sandboxes with a starter account. If you're currently in a free trial you should expect to have these functions until the free trial ends or if you choose to upgrade.