Why I can't follow anymore?

Every request to follow/unfollow someone on Twitter handled by Twitter individually. After every follow/unfollow request, Twitter sends us a response along with the reason if your request not fulfilled at the time.

When Twitter says "You're not allowed to follow anymore," that means Twitter doesn’t like your aggressive follow/unfollow churn, or you've used all your daily follow limits within the Twitter universe. When a user follows too many users in a short period or unfollows them, Twitter marks the user for a period to close-watch him/her for follows/unfollows across the Twitter ecosystem including all 3rd party apps like Circleboom, Twitter mobile apps, and websites.

Twitter has its own way of metrics to decide if you’re eligible to follow anyone on any of its connected platforms. You may hit your “account limitations” or "platform based limitations” either together or individually. But, what we're sure that once you hit the limit, Twitter eventually shuts you off for a certain amount of time to be sure you’re using in designated limits by them. The "follow limit" is not always 400/day, it might be a lower limit for an account. Twitter doesn’t share this info with anyone. if you get an error through from Circleboom, you’ll get one of "follow errors" from Twitter’s own website sooner or later.

Twitter suspends users' account time to time when they do an aggressive follow churn as mentioned on Circleboom TOS and Twitter Rules:

"What is aggressive follow churn? If you decide to follow someone and then change your mind later, you can visit the person’s profile page and unfollow them. Aggressive follow churn is when an account repeatedly follows and then unfollows a large number of Twitter accounts. That may be done to get lots of people to notice them, to circumvent a Twitter limit, or to change their follower-to-following ratio. These behaviors negatively impact the Twitter experience for other people, are common spam tactics, and may lead to account suspension. "

Twitter’s rules bind everyone.

What we suggest is to use tools with nonaggressive intent to strengthen your Twitter account(s). We also recommend pausing follow/unfollows for a few days to let your Twitter account settle down and disappear on Twitter’s radar.