A simple user can't edit the permissions of its own messages on a message board.
How to reproduce :
- Add a new message on a message board's thread
- Try to edit the permissions of the message
=> You have the "You do not have the required permissions." message displayed.
I added the permission "Permissions" to "Power User" on the message just added with my admin account and tried again to modify the permissions with the message owner and it worked. It seems that Liferay doesn't pay attention to the owner permissions.