You get Quality "Read Error" on an OPC tag and the status bar will show "The item is no longer available in the server address space"

If it is a new configuration you want to test, the error is because the OPC server was not turned off when you started the monitor function. In the lower right corner there is an indication that shows whether the server is on (solid green) or off (solid red). You must wait until the OPC server is turned off before testing a change.