Hi @Salashah,
When comparing your redirect URL to our documentation, it looks like you didn’t completely encode all variables such as “&” between query string params. Please closely follow the documentation here.
Below is a correct example of the complete base string:
https://ads.twitter.com/link_managed_account?callback_url=https%3A%2F%2Fmanagingpartner.com%2Flink_account_callback&client_app_id=12345&fi_description=some%20name&promotable_user_id=1&signature=KBxQMMSpKRrtg9aw3qxK4fTXvUc%3D