If you'd like to create a text-only Vertical video, select the Publishing Tools icon from the left rail and select Video Studio. Then, select "Add some text."
You can then enter your text. On the right side, you can use the buttons to change the text's color, background color and transparency, font type, size, box shape and more.
You can also change the slide's background color by clicking "Background" at the top. You can also add more text by clicking on the "+" sign at the top of the video editor.