Traceback (most recent call last):
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\web.py", line 1512, in _execute
        result = yield result
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1055, in run
        value = future.result()
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\concurrent.py"
 
, line 238, in result
        raise_exc_info(self._exc_info)
      File "<string>", line 4, in raise_exc_info
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1063, in run
        yielded = self.gen.throw(*exc_info)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\notebook\services\sessions\handlers.py"
 
, line 73, in post
        type=mtype))
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1055, in run
        value = future.result()
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\concurrent.py", line 238, in result
        raise_exc_info(self._exc_info)
      File "<string>"
 
, line 4, in raise_exc_info
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1063, in run
        yielded = self.gen.throw(*exc_info)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\notebook\services\sessions\sessionmanager.py", line 79, in create_session
        kernel_id = yield self.start_kernel_for_session(session_id, path, name, type, kernel_name)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1055, in run
        value = future.result()
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\concurrent.py", line 238, in result
        raise_exc_info(self._exc_info)
      File "<string>", line 4, in raise_exc_info
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1063, in run
        yielded = self.gen.throw(*exc_info)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\notebook\services\sessions\sessionmanager.py", line 92, in start_kernel_for_session
        self.kernel_manager.start_kernel(path=kernel_path, kernel_name=kernel_name)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 1055, in run
        value = future.result()
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\concurrent.py", line 238, in result
        raise_exc_info(self._exc_info)
      File "<string>", line 4, in raise_exc_info
      File "E:\Anaconda3\envs\fastai\lib\site-packages\tornado\gen.py", line 307, in wrapper
        yielded = next(result)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\notebook\services\kernels\kernelmanager.py", line 160, in start_kernel
        super(MappingKernelManager, self).start_kernel(**kwargs)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\jupyter_client\multikernelmanager.py", line 110, in start_kernel
        km.start_kernel(**kwargs)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\jupyter_client\manager.py", line 259, in start_kernel
        **kw)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\jupyter_client\manager.py", line 204, in _launch_kernel
        return launch_kernel(kernel_cmd, **kw)
      File "E:\Anaconda3\envs\fastai\lib\site-packages\jupyter_client\launcher.py", line 128, in launch_kernel
        proc = Popen(cmd, **kwargs)
      File "E:\Anaconda3\envs\fastai\lib\subprocess.py", line 709, in __init__
        restore_signals, start_new_session)
      File "E:\Anaconda3\envs\fastai\lib\subprocess.py", line 997, in _execute_child
        startupinfo)
    FileNotFoundError: [WinError 2] 系統找不到指定的檔案。

起源是安裝fastai庫時執行conda env update莫名建立了新的環境。
根據解答:

python -m ipykernel install --user