This error is analogous to Agent List Access Denied.
You attempted to edit the MLS Setup or Phone Book Setup on a network version of PC Access. PC Access tried to lock the access control file ACCESS.CTL in the PC Access executable directory. This would let you edit the phone book file PCAWIN.PBK while preventing any other PC Access user on your network from editing it before you finish. However, another PC Access user on your network appears to have locked the access control file. (The error message should show the Agent Name of the user who is already editing the phone book file.) You will have to wait until this user finishes editing the phone book file PCAWIN.PBK for your network copy of PC Access. Then you will be able to edit the phone book file.
If you get this error message repeatedly even though you are sure no other PC Access user on your network is editing PCAWIN.PBK, you may have a "stuck" file lock. If so, follow the instructions for an Internal Error.