I added the isometric_projection.inx and sometric_projection.py to extesion folder.

I do get Axonometric menu which opens but transformation does not work I get error.

C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:11: DeprecationWarning: inkex.localize was moved to inkex.localization.localize.
  inkex.localize()
C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:78: DeprecationWarning: Effect.OptionParser or `optparse` has been deprecated and replaced with `argparser`. You must change `self.OptionParser.add_option` to `self.arg_parser.add_argument`; the arguments are similar.
  self.OptionParser.add_option(
C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:82: DeprecationWarning: Effect.OptionParser or `optparse` has been deprecated and replaced with `argparser`. You must change `self.OptionParser.add_option` to `self.arg_parser.add_argument`; the arguments are similar.
  self.OptionParser.add_option(
C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:198: DeprecationWarning: Effect.affect is now `Effect.run()`. The `output` argument has changed.
  effect.affect()
C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:162: DeprecationWarning: Effect.selected is now a dict in the SvgDocumentElement class. Use `self.svg.selected`.
  if len(self.selected) == 0:
C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:172: DeprecationWarning: Effect.selected is now a dict in the SvgDocumentElement class. Use `self.svg.selected`.
  for id, node in self.selected.items():
C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py:173: DeprecationWarning: simpletransform.computeBBox -> sum([node.bounding_box() for node in aList])
  bbox = computeBBox([node])
Traceback (most recent call last):
  File "C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py", line 198, in <module>
    effect.affect()
  File "C:\Program Files\Inkscape\share\inkscape\extensions\inkex\deprecated.py", line 183, in affect
    return self.run(args=args)
  File "C:\Program Files\Inkscape\share\inkscape\extensions\inkex\base.py", line 131, in run
    self.save_raw(self.effect())
  File "C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py", line 174, in effect
    midpoint = self.getMidPoint(bbox, node)
  File "C:\Program Files\Inkscape\share\inkscape\extensions\isometric_projection.py", line 119, in getMidPoint
    x = bbox[1] - (bbox[1] - bbox[0]) / 2
TypeError: 'BoundingBox' object is not subscriptable

My Inkscape is version 1.1

It seems imho the version is not compatible. I installed Isometric Projection 2.1 and it works with Inkscape 1.1.2:

Have you been here for more Info already? https://inkscape.org/~jdhoek/★inkscape-isometric-projection-21