I'm interested in what version of 6200-PLC5 you have (it's on the Main Screen) precisely what menu you're in, how the RS-232 communications is configured, and which Function Key you press just before the error appears.
I have three suggestions:
1. Make sure you're pointing toward a COM port that is available. If your computer only has COM1, for example, 6200-PLC5 software might give this error if configured for COM2. Be sure also that you're not running a TSR-type program (like Interchange) that might be using up the serial port resource for another purpose.
2. Make sure your serial settings are correct and match the PLC-5 serial port. Usually you just get a timeout, but I think this error is possible simply if the serial port settings don't match. One of the great features in AI-5 and in RSLinx (even in RSLinx Lite) is the ability to connect to a DF1 serial port and Auto-Detect it's settings.
3. There is a file called USER.P23 or maybe USER5.P23 that is in the C:\IPDS\ATTACH directory and holds all the communication channel configuration information. If this file somehow gets corrupted (like switching off the PC when it's open) it can cause that error. Shut down 6200-PLC5 software, delete USER.P23 (and/or USER5.P23) and restart the software. You'll have to reconfigure your preferences, but if that was the problem it'll let the software connect to the PLC-5.