Later does not currently support text-only posts for Facebook, Twitter, or LinkedIn. This is because Later is a social-marketing tool focused on visual content.
While we haven't ruled out introducing this ability in the future, keep in mind that visual-based content can catch your audience's attention and draw them in. For this reason, we encourage our users to post with media for better engagement potential.
That said, If text-only posting for Facebook, Twitter, or LinkedIn is a feature you are keen to see in Later, you can upvote it in the Later Ideas Forum.