in my app the user logged in using phone number auth from firebase , after logged he can add his email from profile fragment , i want to verify this email by sending email link from firebase but the problem always Failed to send the email, i dont know why

I am using email authentication in fireabase for authentication in my andorid applicaiton. I want to give users an unique userId, with which they can login later. Does firebase provide such system and How to implement this.