Due to several reasons you may see a message as “No permission to modify or update” as like a image below.
Don’t worry, it’s just a notice.

First reason: If you directly open a edit page, you will see this message. This is because this page will look for the post to modify but since it is directly opened do not have any reference to it and this notice appears.

Second reason : The current logged in user do not have permission to edit the post. Either this post belongs to someone else or deleted.