“Whenever you use the WordPress control panel to automatically install, upgrade, or delete plugins, WordPress must make changes to files on the filesystem.
Before making any changes, WordPress first checks to see whether or not it has access to directly manipulate the file system.
If WordPress does not have the necessary permissions to modify the filesystem directly, you will be asked for FTP credentials so that WordPress can try to do what it needs to via FTP.”
Solution: In order to find out what user your instance of apache is running as, create a test script with the following content:
For me, it was daemon and not www-data. Then, fix the permission by:
sudo chown -R daemon /path/to/your/local/www/folder
test