Python Scripting not sure whats wrong in my script and why it is not running

The code is to generate 6 unique random numbers from 10 to 99. Once the function is completed it will print the six random numbers on one line with a – between each number. The code

import random

def random6unique():
# list to store random numbers
random_nums = []

i = 0
# looping till 6 unique random numbers are generated
while i < 6:
    # generating a random number
    num = random.randint(10,99)
    # checking if its not present in list
    # then adding it to list and incrementing i
    if num not in random_nums:
        random_nums.append(num)
        i += 1

# printing the numbers in the given format
print(str(random_nums[0]),end="")
for j in range(1,6):
    print("-" + str(random_nums[j]),end="")

print()