Hello Phan, and welcome to E-Learning Heroes. 😊
I'm curious if the issue happens in any course you are working on or just this particular one?
- If you cannot re-create the issue in a new course, I would advise you to import the file into a new one and proceed.
- If it's happening in any/all files, you should conduct a repair of your software.
If you'd prefer to share your project file with our support engineers to investigate what's happening, you can share it privately by uploading it here. It will be deleted when troubleshooting is complete.