Thank you for the question. First of all, please, check that you started monitoring using either “Monitoring” –> “Start” option from the main menu, or clicking “Start” toolbar button.
Also, it is possible that some other software has already opened this COM-port. Please, check that you don’t have any other software which is working with this port at the moment.
However, if you will still having issues with Serial Port Monitor please send your issue to support @ eltima.com and we will be glad to assist you further.