What endpoint are you using?
/2/tweets
What API version are you on?
2
Are you using a library or SDK? Which one?
git/abraham/twitteroauth

What is the issue?
Authenticated user can’t reply to thread even though authenticated user mentioned by author of the thread. Tweet has been restricted by author people they follow or mentioned can reply.

What is the error message?
{“detail”:“You are not allowed to reply to this Tweet as the Tweet author has restricted who can reply to this Tweet.”,“type”:“about:blank”,“title”:“Forbidden”,“status”:403}

Do you have a link to the tweet? Because if the mention is not in the root tweet it may not work. Can the same user reply to the tweet via the website still?