Then the jail is not initiated correctly. You can check that in /etc/passwd. As fasr as I know, you can not even use sftp in a jail in ispconfig, so if the jail would be there, then not sftp login is possible.
In general, I recommend that you use ftps and not sftp. ftps is FTP over a secure TLS encrypted connection which runs over the FTP daemon so that it can benefit from the virtual ftp jails while sftp is a ssh protocol and needs full ssh jails.