Thank you very much. Today when I was saving the project, I got the message saying "there is no enough space in the disk". As I am using VMware Fusion to use Storyline with a Mac, I checked both the disk on Mac and the disk on PC. It turns out to be the temporary files stored in Appdata/Roaming/Articulate that take up a lot of space. The date of those files started from when I used Storyline for the first time, which is February 23rd, 2018.
Is there a setting for Storyline to auto clean those temporary files?