FB> Help says that the syntax is "dir " but I have tried
 FB> everything...

The message you see is coming from your FTP client, not Mystic's FTP server.
"dir" isn't a function of FTP (some FTP clients try to make them work like a
DOS shell or a Linux shell).

I can't provide any more information based on what you've shown so far.  Have
you tried a different client or looked at tried turning on debug level
logging for MIS to see what the FTP server says?

Most of the time people struggle with FTP its because they don't open the
ports on their router to allow the FTP port range.

I would also suggest to use the latest A46 build.

--- Mystic BBS v1.12 A46 2020/08/22 (Windows/64)