Scheduling tweets are especially important when it comes to those who want to schedule multiple tweets at different timings. Not only can you schedule your tweets, but you can also save unsent tweets as drafts. However, unsent tweets will only be accessible on other desktop/mobile web and not on the mobile app itself. This means that whatever tweet draft that you save on the web will not be synced to your mobile app.
— Twitter Support (@TwitterSupport) May 28, 2020
When you click on the Tweet composer, you’ll see the “Unsent Tweets” section on the top right; clicking on that will redirect you to your drafts and scheduled tweets. You can either select those tweets to edit them or click on the edit button on the top right to delete unwanted tweets.
These new features were tested back in November 2019, but not everyone had access to it then. Still, the inability to sync your unsent tweets on the mobile app and web app is a bit of a disappointment; perhaps Twitter might do something about that in the future.