When performing a migration, you get this error message:
The page cannot be copied because it contains modern web parts that are not supported by the destination library.
Modern pages associated to Microsoft's "New Experience" are not supported for migrations to on-premises versions of SharePoint. In order to use them, the Site Pages feature must be activated to enable the Site Page content type.
These errors can occur when trying to migrate a page to an environment other than SharePoint Online and Office 365, or when the content type mapping is not Site Page.
If copying to Office 365, make sure that the Site Page feature is activated at the site-level and that the site content type Site Page is available.
You can do this by selecting the site in Explorer, and clicking Edit in the Quick Actions menu. Once there, select Manage Site Features in Transformations, select Site Pages, and select Activate from the drop down menu.
If the feature is already activated, make sure that the Site Page content type is available on the destination library and review the content type mappings to ensure that the source content type is correctly mapped to Site Page at the destination.