You first need to install the corresponding library files for your Arduino. ModbusMaster.h

Thank you for your reply, Chris. Anticipate update if there are any new developments. If it's an issue with my settings, please let me know as well. T...

@chris-annin Thank you for your prompt reply. Yes, I am using version 5.1. I have also tried versions 4.3.1, 4.4, and 5.0, but the issue remain...