Facing issues while installing rasa_core
I faced the same issue and able to install rasa_core after resolving dependencies fast.
Please try below:

First install twisted
pip install Twisted
Then, install rasa_core     
pip install rasa_core
I am trying to install rasa_core in my python by using  !pip install rasa_core; command.
But i am getting an error :
Below is the error :
Failed building wheel for Twisted
  The scripts freeze_graph.exe, saved_model_cli.exe, tensorboard.exe, tflite_convert.exe, toco.exe and toco_from_protos.exe are installed in 'C:\Users\user\AppData\Roaming\Python\Python36\Scripts' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Command "c:\programdata\anaconda3\python.exe -u -c "import setuptools, tokenize;__file__='C:\\Users\\user\\AppData\\Local\\Temp\\pip-install-fot9mu3e\\Twisted\\setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record C:\Users\user\AppData\Local\Temp\pip-record-vp3wq_7u\install-record.txt --single-version-externally-managed --compile --user --prefix=" failed with error code 1 in C:\Users\user\AppData\Local\Temp\pip-install-fot9mu3e\Twisted\
Could anyone please help me.