I'm looking for a shortcut to access the file properties for files on my desktop. The official website says that the shortcut is for this is Alt+Enter, but it doesn't work because this key combination has the same effect as just pressing Enter.
The shortcut works if the file/folder is inside a folder, but it doesn't work if the file/folder is on the desktop.
Thank you for your time.
Since you are using the GNOME desktop environment, you should know that GNOME removed desktop icons since version 3.28 and Ubuntu enables them using the Desktop Icons extension. This extension has very limited options though and hasn't seen much development.
The reason that you can't use Alt+Enter to display file properties from the desktop is simply because this option isn't implemented in the Desktop Icons extension.
The only alternatives to use this functionality for the moment is to access your desktop files from within Nautilus, or use another file manager (e.g. Nemo - see How to set Nemo as the default file manager in Ubuntu?) to handle desktop icons, or use another desktop environment that offers this functionality.