There are three scenarios where the "Delete page" option may be greyed out. Firstly, if the file only contains one page, deletion is not possible as at least one page is necessary for the file to exist. Secondly, the file may be protected, restricting you from deleting pages. Please navigate to File->Properties->Security to verify if all permissions are granted. If all permissions are granted, then the third scenario applies - the file may be a PDF/A standard file. In this case, you can enable editing by navigating to File->Preferences->Documents, and selecting "Never" for the option "View document in PDF/A mode". After restarting the program with the PDF file opened, the "Delete page" option should be enabled.