Follow the steps to make your twitter account private in a mobile app.
-Open the twitter app on your mobile device.
-Tap on your profile picture and find the Setting and Support then Settings and Privacy Option.
-On the same page click on Privacy and Safety.
-Then click on Audience and Tagging.
-Then Click on the protect your posts option.
By following these simple steps you can easily make your twitter account private.
https://techyvibe.net/how-to-make-twitter-account-private/