That error indicates that the API resource you tried to access is not a valid API endpoint. In this case, you’ve missed the “/cards/” in the URL.
A query to see all cards on an account would use the GET accounts/:account_id/cards/lead_gen endpoint, and would look something like this:
https://ads-api.twitter.com/0/accounts/abc123/cards/lead_gen
Note that there is a bug in that documentation, and you cannot limit the results of the cards by line_item_id, since that is not an attribute of a card. We’ll fix that documentation bug.