Your script does not wait so it exists immediately.
You can either add an infinite loop at the end, or run it with micropython -i which runs the script and then opens the interactive REPL instead of exiting.
As @embeddedt already mentioned - you cannot use pdb (or any other Python tool) with Micropython.
As far as I know there is no Micropython debugger. sys.settrace is implemented so it’s possible to get a trace of the run. But Micropython core is inherently missing the ability to query/reflect local variables so a true debugger cannot be implemented for now.