Why do I get this error message when trying to enter anything on either of a pair of linked UP700's? The machines use the plastic keys to log in the operators and have no Clerk# key on the keyboard. Any suggestions would be much appreciated.

I'm assuming you don't have a service key.
With your MA key, go to PGM2 on the master machine. Then turn the machine off at the side. Hold down the JRNL FEED and RCPT FEED keys at the same time, keep holding them as you turn the power back on. This will do a service reset and get the master out of what it was stuck in. Don't forget to do an 'open store' after you've done this.
If this doesn't work, then this function has been disabled in the service presets and you will need an SRV key.