scraping multiple pages of a website.

[buran]

Larz60+ has done wonderful job writing this for you, but I think 'it's too complicated for something that can be done with couple of lines (i.e. OOP, etc is overkill)
first of all - your code. The problem is on line#11.
Here is it with some small changes

import requests
from bs4 import BeautifulSoup
from string import ascii_lowercase
 
base_url  = 'https://www.usa.gov/federal-agencies/'
for letter in ascii_lowercase:
    url = '{}{}'.format(base_url, letter)
    print(url)
    resp = requests.get(url)
    soup = BeautifulSoup(resp.text, 'html.parser')
    for ul in soup.find_all('ul', {'class' : 'one_column_bullet'}):
        print(ul))

Now the nice part
If you inspect the page and what it loads you will notice that it gets all the information as json
so

import requests

url = "https://www.usa.gov/ajax/federal-agencies/autocomplete"
resp = requests.get(url)
print(resp.json())

if you want you can save the json resp as file. Anyway, you get all 548 agencies and the respective url in one get request as a json file.