Here is more progress on the issue:
Apparently, one must choose one of the radio button (Enable Phone or Email verification). Then the form appears to be saved. (too bad we can’t post pictures here!)
However, in my scenario, users are registered/created thorough API, by having their username as their mobile number and we will use the plugin for OTP login only. I now don’t see the “Error password field empty” error, but instead a new page appears after sending the form : “There was an error in sending the OTP. Please enter a valid email id or contact site Admin.” What’s the logic behind of this form?