This error probably means you are running PC Access over a network, another user is running another instance of PC Access on another network workstation, and that user has selected the same agent from the Agent List window as you did. (The error message should show the Agent ID the other user has already selected.) Normally you should not try to select the same agent that another PC Access user on your network has already selected.
If you must run two instances of the same copy of PC Access with the same agent, you will have to check the Disable scroll back box on the Defaults Setup window. (Even so, you will not be able to Call a BORIS MLS that disallows simultaneous logins from the same agent.)
If a copy of PC Access on your network crashed while its user had selected this agent, PC Access may incorrectly "think" someone has currently selected this agent. To correct this, your System Administrator must highlight this agent on the Agent Setup window and press the Reset button.
If you are not running PC Access on a network, follow the instructions for an Internal Error.