A little late I know, but I found this question after having the same problem myself.
I discovered that you must use the twitter user's ID and not the screen name, in a follow
request.
There are a few websites which claim to offer a quick twitter username -> ID search, but none of them worked for me. Instead, I found that if you view the source code on a page containing a tweet from the user and search for data-user-id
, you will find it listed as an attribute in an HTML element (usually the second or third occurance of this contains the ID).
Hope this helps someone.