Use of Thonny for Debugging

I have created a short programme in MicroPython using Thonny to control LEDs via a Plasma2040 board. The version of Thonny on my MacBook is 3.3.13 and my MacBook is running Big Sur 11.6.2. I have set the interpreter in Thonny to be "MicroPython ( Raspberry Pi PICO). I can launch and run the programme from Thonny but all of the debug buttons are greyed out and cannot be used. Any thoughts on how I can rectify this situation as I would like to be able to single step the programme and see exactly what is going on.