Gnucash said "This account register is read-only."

When I started up GnuCash, I saw a dialog box saying "This account register is read-only".

This account may not be edited. If you want to edit transactions in this register, please open the account option and turn off the placeholder checkbox.

The error message is not good because it is not obvious that what "This account" mean. A post on Gmane (originally posted comp.gnome.apps.gnucash.user newsgroup) explains it. http://article.gmane.org/gmane.comp.gnome.apps.gnucash.user/24490