Trying to add a new directory in the root of my server

Question

just moved from shared hosting to a dedicated server, trying to create two new directorys in the root of the server but I get this when I use the Plesk control panel

Error: Unable to create directory /other-files: filemng: Error occurred during /bin/mkdir command.

So I tried executing this

$old_umask = umask(0);
$return = mkdir('../other-files', 0770);
umask($old_umask);
var_dump($return); 

which returns false

I've downloaded putty but I've never used SSH commands and to my understanding I think I need to create the folder as a root user, but I really don't want to do something wrong and I don't think I can log in as the root user, but I can execute a su command which seems to be what I need to do... any help?