Similar to bug #
PHPBB3-9107 I am unable to set the parent forum and not only this but the you can't copy permissions either.
I first noticed the issue in phpBB 3.0.8 (installed by cPanel), but then upgraded to phpBB 3.0.10 to see if the problem persisted, which it did.
I checked my files against the official phpBB 3.0.10 release and they show as identical so I can only assume its some kind of database issue.
The drop down list for both "Parent forum" and "Copy permissions from" are greyed out and you cannot click on any of the options. They are actually set in the HTML to disabled.