Twitter has restricted advertisers from targeting followers of another account (that isn't your own) - you can only use a targeting option called Similar Followers.
The workaround we use is to recruit a third party to scrape the followers from the handle you want to target. We then take the excel file, remove everything except the Twitter handles, then use this to create a Tailored Audience. We then inclusion target this Tailored Audience.
Workaround 3rd party options:
Followerwonk follower report (download to excel with a free trial):
Simplymeasured follower report (download to excel on the right):