FTP is a pretty loose protocol. Some servers implement features that others do not. Depending on your server, "LS" may not be supported:
5. DECLARATIVE SPECIFICATIONS
5.1. MINIMUM IMPLEMENTATION
In order to make FTP workable without needless error messages, the
following minimum implementation is required for all servers:
TYPE - ASCII Non-print
MODE - Stream
STRUCTURE - File, Record
COMMANDS - USER, QUIT, PORT,
TYPE, MODE, STRU,
for the default values
RETR, STOR,
NOOP.
The default values for transfer parameters are:
TYPE - ASCII Non-print
MODE - Stream
STRU - File
All hosts must accept the above as the standard defaults.
RFC 959 - File Transfer Protocol As a matter of fact, I don't see "LS" at all in the RFC.
Try using a command-line FTP client to log in. Typing in "help" may get you a list of valid commands for that server.