Why the plugin cannot overwrite a file? What are the necessary permissions?

The Unix systems (that includes Linux in all its flavors and distributions) have for each file permissions and ownership (read more here).

We've seen that many times, when a website is moved from one hosting provider to another, while the permissions are correct the ownership of a file is not correctly kept. For example, a file can have the correct write permissions but the file owner is "guest" instead of "www-data". This means that trying to replace a file when using a plugin like Enable Media Replace or ShortPixel Image Optimizer will fail.

The solution to this is usually to contact your hosting provider, after moving a website from one hoster to another, and make sure that all your files have both the correct ownership and the correct permissions. Once this is fixed you'll be able to replace files using plugins like the ones mentioned before.