Phpmyadmin: error creating table

$info_query = $PowerBB->DB->sql_query("SELECT * FROM " . $PowerBB->table['info'] . " WHERE var_name='prefix_installed'");
$info_row = $PowerBB->DB->sql_fetch_array($info_query);
if (!$info_row)
{
 $PowerBB->DB->sql_query("CREATE TABLE `".$PowerBB->prefix."prefix` (
`id` int(9) NOT NULL AUTO_INCREMENT,
`prefix` varchar(100) NOT NULL,
`forum` INT( 9 ) NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;");
$PowerBB->DB->sql_query("ALTER TABLE " . $PowerBB->table['subject'] . " ADD `prefix` VARCHAR( 100 ) NULL");
$PowerBB->DB->sql_query('INSERT INTO ' . $PowerBB->table['info'] . " SET var_name='prefix_installed',value='1' ");
}

Error Number:0
Error Query: CREATE TABLE `pbb_prefix` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`prefix` varchar(100) NOT NULL,
`forum` INT(11) NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;