FTP reply code 501 means the server understood the command verb but the parameters that followed are syntactically invalid. The command itself is fine — the arguments are wrong. Common triggers include invalid file paths (containing illegal characters), malformed PORT command parameters (wrong IP/port format), incorrect TYPE parameters (something other than A, I, E, or L), or invalid arguments to SITE commands. The client should fix the parameters and resend the command.
The path argument contains characters the server does not accept, or uses the wrong path separator (backslash vs forward slash). Some servers reject paths with spaces unless quoted.
The PORT command requires six comma-separated numbers (h1,h2,h3,h4,p1,p2) representing the IP and port. If the format is wrong or the values are out of range (0-255 for each octet), the server returns 501.
The TYPE command only accepts A (ASCII), I (Image/Binary), E (EBCDIC), or L (Local byte size). Sending TYPE with any other parameter results in 501.
Review the exact parameters being sent. For file paths, use forward slashes and avoid special characters. For PORT, ensure the format is correct: PORT h1,h2,h3,h4,p1,p2.
If the file or directory name contains spaces, try quoting the path or escaping the spaces. Different servers handle quoting differently.
Try the command with a simple filename (no spaces, no special characters) to confirm the command works. Then gradually add complexity to identify which character causes the error.
The server did not recognize the FTP command. It may be misspelled, malformed, or unsupported.
The file or directory operation failed. The path does not exist, or access is denied.
The FTP commands were sent in the wrong order. The server requires a specific command sequence.