If you are using a cloud service (Google Drive, DropBox, OneDrive, etc.) to back up your projects, your files are moved from the local drive and uploaded to the cloud.
So, when you open the project, CreateStudio is unable to find those files.
When this happens, you might get this message:
Here's what you can do:
1. Download the project to your local drive, then open the project in CS as usual.
OR
2. Keep the project in the cloud, but also make it available offline.
How to make files available offline