If your tweets are failing to send, and you're seeing the error below, this is due to your Twitter account being locked. This error message comes directly from Twitter.
Oops, it looks like your Twitter profile was locked when we tried to publish this. Please visit twitter.com to unlock your account and then retry the post.
To unlock your account, please log into Twitter and send a tweet directly from there. You may be required to complete additional security steps at this point. Once your account has been unlocked, your tweets from Buffer should start successfully sending again.