You can't delete these keywords because you have limited access or are trying to delete a keyword in a dynamic view that you created.
If you have limited access
If you wish to delete the keywords, one of the admins must change you to admin user type or delete the keywords for you. Here is an article on how to add and modify user access to the account.
If you are trying to delete the keyword from a dynamic view
You can delete the keywords from the All Keywords view. This is a safety measure, and in most cases, the views are created based on dynamic filters. An option to delete these views or keywords within the views could cause some keywords to be permanently deleted accidentally.