When you connect using PC5250 emulation, underneath the covers it is using AppAdmin to query the OS/400 for the user settings of the user profile you use to connect to OS/400. AppAdmin uses the remote command to query OS/400. Make sure the AS-RMTCMD daemon job is up and in a listen state by doing a NETSTAT *CNN command. You should see AS-RMTCMD in listen state on port(s) 8475 and 9475 depending if you are using SSL. For a complete list of Client Access TCP/IP ports see
https://www.ibm.com/support/pages/tcpip-ports-required-ibm-i-access-and-related-functions
If the PC is remote to OS/400 and is going through a firewall and/or routers, make sure the 8475 and/or the 9475 port is open.
Also check the service pack level of Client Access - if there is no service pack installed, this message WILL appear. To check the level, open the control panel, click on Client Access (or iSeries Access) - the general tab will list Version, Release, Modification, and Service level. If the word is None, there is no service pack installed.