You cannot enter half-width characters in the Jupiter Notebook terminal.

I had the same symptoms myself, but when I opened it from Vivaldi in a different browser instead of Safari, the problem did not occur.This is MacOS Sierra 10.12.6.

I couldn't use half-width mode with Japanese input, but ABC worked well.

By the way, ABC can be added from the + button on the System Preferences/Keyboards/Input Sources screen.

I'm having trouble with the same symptoms, but if you type in half-width in another text app and copy it, it will work. This is the first aid for now.

Is the CapsLock key pressed?

I have the same phenomenon.
The jupytyer notebook allows you to execute commands from cells, so
I think it would be good to use !pip install in the cell.