Quoting https://winscp.net/eng/docs/ui_login_ftp :
Use Post login commands to specify set of FTP protocol commands that should be issued automatically just after login. This is typically used to issue commands that change session configuration or state. Only commands that require no interaction with client can be used.
getis not FTP protocol command. Equivalent is
RETR. But still it's not a command that "require no interaction with client".