Why my hidden photos still does not open?

Where are you looking for the album?

It will appear in the "Library". section of the sidebar, not among the albums. The name of the album may be different, depending on the primary language on your Mac.

Hi

it’s because you’ve not hidden any photos

ctrl click a photo and select hide. Then the album will appear and the hidden photo will be in it.

To make the hidden album visible in the sidebar, use the command View > Show Hidden Photos Album.