Debugging nanoflow is still an idea, may be you can up vote here. https://forum.mendix.com/link/ideas/1184 .
However, if you are looking for debugging the js, try adding debugger statement in script and inspect in console.
There are 2 way. If you want to debug 1 specific action add inside the action a line debugger; https://www.w3schools.com/jsref/jsref_debugger.asp This will open the devloper console and break on the the specified line.
If you want to do more general debugging. Your action have to runs first, then there will be jsaction.js file available in the developer tool files. Add your desired breakpoints and re-run your actions.
Unfortunately there is no debugging of nanoflow in the same way there is van java actions or microflows.
Upvote for this feature on the idea forum.