This error occurs when the page you are trying to delete is in use by a Fundraiser. When a page is in use by a Fundraiser, the page cannot be deleted unless the associated Fundraiser page is deleted first. 

See How to delete a Fundraising page for more information. 

Note: if you are unable to complete the steps in the article above, you may consider archiving the page by editing the page name to Archived or Obsolete, for example, and then create a folder under Site Explorer>Pages & Templates, for example Archived or Obsolete, and move the archived pages to this folder.


Additional Resources
Error: X page could not be moved to Deleted Pages/Templates because it is in use in site configuration - when trying to delete page
Error: Page could not be moved to Deleted Pages/Templates because it is in use by one or more other components - when trying to delete page in Site Explorer