This can happen if you don't have Create / Publish permission for that page.
You can check permissions if you go to Admin mode / Set Access Rigths.
Hope this helps!
It is the same for any user, even when logged in as a user in the Administrator group. It doesn't appear to be a access rights issue.
I want to create a block under the "For This Page" folder in the Blocks widget but when I click "Create Content" > "New Block", and the "Select Folder" modal displays, if I click the "For This Page" folder, the "Select" button remains greyed out.
I am able to create blocks under the "For All Sites" folder.
Am I missing some permissions setting somewhere? I didn't think this would be that difficult but I can't find any mention of this anywhere in the documentation.