param is missing or the value is empty: user

該当コード

class Api::UsersController < ApplicationController
.
.
.
  private

  def user_params
    params.require(:user).permit(:email, :password, :password_confirmation, :name)
  end
end

解決方法

class Api::UsersController < ApplicationController
.
.
.
  private

  def user_params
    params.permit(:email, :password, :password_confirmation, :name)
  end
end

参考記事：【Rails】param is missing or the value is empty:について - Qiita