Hi,
seems that the pdf24.exe background process is not running. Please check that with your task manager. There must be a running pdf24.exe task.
If there is no pdf24.exe then the PDF printer will not work. Restart the pdf24.exe to get the PDF printer to work.
By starting the PPDF24 Editor the pdf24.exe is started implicitly because it is needed by the editor, but the pdf24.exe will be closed by clsing the editor if the editor has started the pdf24.exe.