Sign in to app.speechify.com using your Speechify account credentials, preferably in a Chromium browser.
The home page displays your Speechify library.
Click on the three-dot menu on the file that you want to move.
Select 'Move' and choose the folder that you want to move it to from the drop-down menu in the pop-up. Click on 'Move' to move your file.
Learn how to create a new folder here.