Debugging an Arduino Uno is possible via either simulating it (simavr) or running avr-stub on it and communicating with it via the normal serial port (this will also be the needed debug_port). We havea plenty of documentation for that in the post and documentation linked at How to debug on Arduino mega 2560 - #2 by maxgerhardt, simavr — PlatformIO latest documentation, Arduino Uno — PlatformIO latest documentation, etc.