Hey Kumyoung Park 

I suspect, you have your project location but the actual project files are placed elsewhere.

To help troubleshoot this, please can you go to the Project Folder as understood by Trados Studio.
Then ensure you see the target folders and be sure that within these folder you have the working sdlxliff files.

The only other time I am aware of this happening is if you are working on a collaborative project, example GroupShare.
Or your project was a package you received, and now you want to send it to someone else. 

Please confirm if your the origin of your project so we can guide you further 

Lyds

Dear Lydia,

Thank you for your reply!
The project folder was in the right place, and there were no problems.
After uninstalling Trados Studio and reinstalling it, I sold it, and everything worked fine.

Kind regards,

Kumyoung Park , Wonderful