There can be many reasons as to why a user did not receive the login code from Telegram. They are,
User has entered the incorrect phone number.
Telegram sends the login code to the Telegram account of the phone number provided by the user during the sign-in process. Therefore, If the number entered is incorrect, the user will not receive the login code.
** During the sign-in, Make sure you enter the phone number in international format (with the country code).
User has attempted to login too many times.
Telegram usually restricts users from logging in to their telegram account when there are too many login attempts.
Users can confirm if this is why they didn’t receive the login code by trying to login to the account with Telegram web in an Incognito window/ Private window.