I’ve seen this error and only once was it on the iSeries side. Verify the FTP server is active on the iSeries:

Check the subsystem QSYSWRK for a QTFTP* JOB under the user QTCP. If not start the service

There may be a PC side firewall that restricts port 20 for the data connection. End and remove the PC side firewall.

If your client is allowed to make outgoing connections on any port,
and only incoming connections are restricted, then use passive
mode. (This is very common at companies with an intranet)

If the FTP server has the firewall, and it’s allowed to make
outgoing connections on any port, but incoming is restricted
to port 21, then use standard mode (non-passive).

If your LAN is set up to strictly only allow certain ports in
both directions, you may need to set up an FTP proxy so that
you can bypass the firewall.