This is due to the lower version of IBM client access in your PC. Solution to fix this
Go to Task manger , In process tab, you can see the image name PCSWS.EXE & PCSCM.EXE will be running over there. Kindly end all the file and then open the client access application now, It will open and connect to the AS/400 server. You dont need to restart your PC to fix this issue.