Switching layouts is disabled in all modes in empty notebooks.
So, if I create a notebook and happen to be in preview-only mode, I cannot create a new note because the only way to create a new note is in the edit mode. So, I have to switch to another notebook with some notes, switch the layout and switch back to my new notebook.
Also, I did not check, but what if this is the ONLY notebook and it is without notes?
I can confirm that this happens on both Windows and Linux. You can toggle through the layout with the keyboard shortcut of Ctrl+L but the button is grayed out.
Steps to reproduce:
In an existing note, change the layout to view only
Create a new notebook or delete all notes in an existing notebook
Create a new note
If you create a name/title for the note then it will enable the layout button since there is now a note that exists within the notebook.