Does the renaming work if you wait a minute and try it, again? LR might be busy building a preview or writing metadata to the photo or its sidecar and won’t let you rename the photo until it is done doing that.
If waiting doesn’t change the situation, then if you are on Windows, right-click on the thumbnail of a troublesome photo in the thumbnail bar at the bottom and choose Show in Explorer, and if that works, then right-click on the photo in Explorer and choose Properties and look at the Attributes at the bottom of the General tab, looking to see if the Read Only attribute is checked.
If right-clicking on the photo in the thumbnail bar gives an error, then right-click on the folder that contains the photo in the left panel of Library and choose Show in Explorer and find the photo by scrolling down in the Explorer window and right-click and choose Properties then look at Attributes to see if Read Only is enabled.
