First, use the
chgrp command instead of chown and that will work.
In the case of using chown.....For security reasons in most Linux contexts, any ownership change is restricted to the root user eventhough you are marked as the owner of the file, directory, etc or not. In one case, this is to prevent users from evading quotas by setting the file permission bits to 777 and changing the ownership of a file to some other unknown user and eating up their quota.
So using the
chown command in a user context, especially where the ownership is not changing should not be used just to change the group of a file.
NOTE: You must be a member of the group you are attempting to change the file to. This can be verified by
id -a. If you are not in the group, you will get this message eventhough you are the owner of the file.