Forbidden…..You don't have permission to access…

I have just created a new website with the Twenty Thirteen theme. I changed the header image, and removed the comments option on the initial pages.

I then tried to alter the name of the website (for the header) and got the following error message:

You don’t have permission to access /wp-admin/options.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Could anyone please explain what I am doing wrong?