sadly it looks like this module is way out of date and not updated…

there was another user here creating a new fitbit module

see here
https://forum.magicmirror.builders/topic/12029/what-is-the-procedure-for-reviving-a-dead-module

[09:34:25.018] [LOG]    Whoops! There was an uncaught exception...
[09:34:25.020] [ERROR]  TypeError: PythonShell.run is not a function
    at Class.setCreds (/home/pi/MagicMirror/modules/MMM-fitbit/node_helper.js:32:15)
    at Class.socketNotificationReceived (/home/pi/MagicMirror/modules/MMM-fitbit/node_helper.js:18:9)
    at Socket.<anonymous> (/home/pi/MagicMirror/js/node_helper.js:113:11)
    at Socket.emit (events.js:182:13)
    at /home/pi/MagicMirror/node_modules/socket.io/lib/socket.js:528:12
    at process._tickCallback (internal/process/next_tick.js:61:11)
[09:34:25.026] [LOG]    MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
[09:34:25.029] [LOG]    If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues
[09:34:25.136] [LOG]    Initial run request recieved.
[09:34:25.139] [LOG]    Running getData.py
[09:34:25.143] [LOG]    Whoops! There was an uncaught exception...
[09:34:25.146] [ERROR]  TypeError: PythonShell is not a constructor
    at Class.getData (/home/pi/MagicMirror/modules/MMM-fitbit/node_helper.js:43:25)
    at Class.socketNotificationReceived (/home/pi/MagicMirror/modules/MMM-fitbit/node_helper.js:22:9)
    at Socket.<anonymous> (/home/pi/MagicMirror/js/node_helper.js:113:11)
    at Socket.emit (events.js:182:13)
    at /home/pi/MagicMirror/node_modules/socket.io/lib/socket.js:528:12
    at process._tickCallback (internal/process/next_tick.js:61:11)
[09:34:25.148] [LOG]    MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
[09:34:25.150] [LOG]    If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues

This is my output of npm install (usually the log for npm audit fix too):

pi@magicmirror:~/MagicMirror/modules/MMM-fitbit $ sudo npm install python-shell -f
npm WARN using --force I sure hope you know what you are doing.
npm WARN saveError ENOENT: no such file or directory, open '/home/pi/MagicMirror/modules/package.json'
npm WARN enoent ENOENT: no such file or directory, open '/home/pi/MagicMirror/modules/package.json'
npm WARN modules No description
npm WARN modules No repository field.
npm WARN modules No README data
npm WARN modules No license field.

+ python-shell@1.0.8
updated 1 package and audited 391 packages in 3.359s

1 package is looking for funding
  run `npm fund` for details

found 0 vulnerabilities

I was able to solve this by invoking npm init and generating a new package.json file. Now, the output to installing pythoin-shell was

pi@magicmirror:~/MagicMirror/modules/MMM-fitbit $ sudo npm install python-shell -f
npm WARN using --force I sure hope you know what you are doing.
npm notice created a lockfile as package-lock.json. You should commit this file.
+ python-shell@1.0.8
added 1 package from 1 contributor and audited 1 package in 1.4s
found 0 vulnerabilities

No cigar, I still get the above error on npm install

the pyhton-shell module dependency wasn’t installed for it…
from the code in node_helper.js

var PythonShell = require('python-shell');

to fix do
change to the MMM-Fitbit folder

npm install python-shell

the author should have provided a package.json to handle this, with instructions for npm install
there was a security fix added in the npm code, if a module (in node_modules) is found, but not referenced by a package.json dependency, that module is removed (in case someone is trying to sneak stuff into the runtime to steal data)

this will happen to you again