name 'tel' is not defined

Соблюдайте отступы:

import json
from random import choice
def gen_person():
    tel = ''

    nums = ['1' ,'2','3', '4' , '5']

    while len(tel) != 7:
       tel +=  choice(nums)

    print(tel)

def main():
   gen_person()

if __name__ == '__main__':
   main()