Why do I keep getting '429 Too Many Requests' for put requests to add members to a list on Twitter's API? – Twitter API

I am using the following Tweepy code to access the Twitter API endpoint https://api.twitter.com/1.1/lists/members/create.json

It seems that I cannot add more than about 80 members in a given day, but the rate limit is stated to be 75 / 15 minutes. I have my requests throttled to be 50 members every 1000 seconds (16.7 minutes).

Code:

members_to_add = groups['ct']
list_id = '1520581139982868483'
for member in members_to_add[1:2]:
    client.add_list_member(id=list_id, user_id=member)
    print(member)
    time.sleep(1000)

Is there another limit I am hitting that I don’t know about?