judging from the environment name UNO it seems to me that you want to debug an Arduino Uno. Note that
PIO only has simavr, an AVR simulator, built in as a debugger on simulated hardware. it can still fully debug a program, though. It should have launched there, but somehow hasn’t – I’ll investigate.
if simulation is not wanted, the Uno needs additional hardware to be debuggable, e.g. via the debugWire protocol with an AVARICE debug probe or similiar. Not debugable (with line-by-line stepping etc.) with just the Uno and its built-in serial port itself
PIO does not yet have integrated support for debugging realHW regarding AVRs (unlike Atmel Studio)