Why?

[DEVONKVAN]

Why is this happening?

[Test Screen Name]

Administrator: this post is about Illustrator, please move it to the Illustrator forum. Original poster: please provide detailed information. System and Illustrator version, where the files are stored, whether this is a new problem, what feature you are requesting, the connection to file sync, what changed etc. 

[AxelMatt]

Moved to the Illustrator forum

[Ton Frederiks]

Where do you save your files?

Can you move the file to your desktop and try to open it?

[meganchi]

Can you look at the file properties? Is it set to read-only?

Were you downloading the file? Sometimes an error can happen if someone tries opening the file when it wasn't through downloading.