FTP reply code 551 indicates the server aborted the requested action because the file's page type or structure is not recognized. This is a legacy error related to the STRU P (Page structure) transfer mode, which was designed for systems that organize files into pages (like some mainframe filesystems). In modern FTP usage, this error is extremely rare because virtually all transfers use STRU F (File structure) with MODE S (Stream mode). If you encounter 551, you are likely interacting with a legacy system or your FTP client is requesting an unsupported file structure.
The client or server is attempting to use page-structured file transfers (STRU P) which this server does not support. Modern FTP servers only support STRU F (file structure).
The server is a legacy mainframe or minicomputer system that uses page-structured files, and the client is not sending data in the expected page format.
Send STRU F to switch to standard file structure before attempting the transfer. This is the default and most widely supported structure.
Send MODE S to ensure stream mode is active. Combined with STRU F, this is the universal default that works on all FTP servers.
The file or directory operation failed. The path does not exist, or access is denied.
The command is recognized but a specific parameter value is not supported.
The FTP server recognized the command but has not implemented it or has disabled it.