Why the "Lock Protected Sections" option is disabled (gray out) in Windows 10 OneNote?

​Question:

I want to protect some notes with password in OneNote. But the option is disabled. When I right-click on a section, I see the "Lock Protected Sections" option is gray out.

Reason:

OneNote 17 (Win 10 App) cannot encrypt and password protect any unprotected sections.

To enable "Lock Protected Sections" option, you need to:
    

1. Encrypt and password a section in OneNote desktop version.
   As of now the OneNote 17 (Win 10 App) cannot encrypt and password protect any unprotected sections. You need another version (e.g. OneNote 2016 free version for Windows or Mac version) to do that. The menu entry "Lock protected Sections" is NOT meant to encrypt any unencrypted section (note the "protected" in the command, meaning "already protected").
2. What the OneNote 17 can do is decrypt / open an existing password protected section. This section is then "unlocked". It remains unlocked for a certain time (I think 10 mins.). If you don't edit the note in that time it gets locked (encrypted, closed, hidden) again automatically. 
3. Now let's assume you want to leave your device unattended but the encrypted note is still unlocked because the 10 minutes are not over yet. In that case you may choose the "Lock protected section" command to manually and immediately hide the protected section (in fact ALL currently unlocked protected sections).

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So if there is no protected section unlocked at the moment, the command is greyed out as there is nothing to close/hide. As soon as you unlock an existing protected section by entering its password that command gets available.