How to change the Slug or page URL of my page?
You have created a page and the slug name doesn't match the page name. It has an undesirable "-2" which is added. You are trying to remove it but you can't.
Most often, the reason is that your permalink is already in use.
Check the permalinks of your pages and beware that the permalink might be already used by a page in the trash. Make sure to check in your trash section of your pages/posts.
Another case that it won't work is because you are trying to use a slug name that is dedicated as a parent of a post type. Permalinks: "/discography", "/event", "/videos" are dedicated. You cannot use them.
So if you have created a page called "videos", the default permalink will be "/videos-2". You can't use "videos". You can use another slug name as "video" or "-videos-" Or you can also make your page a child page, so the permalink will change. (ex: "/medias/videos")