Why can't I press enter to start a new line of text?

Not sure if you've already tried this - I didn't see it noted in any of the comments above

Have you tried holding Shift when hitting Return/Enter? This is typically used in other software as a soft line break, and in some software it's the way to indicate to the software that you want a new line rather than executing/initiating a command (similar to how commenting on Facebook will send the comment with Return/Enter, but Shift+Return/Enter will create a new line so you can continue typing)

Press Return instead.