I suggest that you first verify the ownership of the file/

With a SSH terminal, you can enter
ls -l
that list all the files and folders and also display the symbolic links values

To change a permission of a file or folder, you can use
chmod 755 <file or directory>
or for a recursive
chmod -R 755 <directory>


If you want to change the owner of a file or folder you can use
chown owner:group <file or folder>

Perhaps you should ask to the administrator of your server to help you verifying the ownership and permission of your server.
If you want that we have a look on your server, this is also possible with out billable support that you can order at
www.jms2win.com/en/joomla-multisite?page...35&category_id=1

For which folder/directory should i change permissions and ownerships rights?

still don't know how to solve this problem

is it possible to have a discount? we are students, we can't afford 75 euros per hour