error/admin/sectionerror

The error means that a new admin page 'foo' (the one you're developing right now) is not registered with the Admin menu. It is triggered with the call:

admin_externalpage_setup('foo');

Solution is to add the registration code to admin/server.php:

if (file_exists("$CFG->dirroot/$CFG->admin/foo.php")) {
    $ADMIN->add('server', new admin_externalpage('foo', "Foo Admin Component", "$CFG->wwwroot/$CFG->admin/foo.php"));
}