Fatal installation error: Call to a member function add_cap() on a non-object

Apparently your site does not have a role named ‘administrator’ defined. I’ve never seen that before. To work around it, you can temporarily add these lines at the top of function SetAdminCapability in includes/manager.php:

add_role('administrator', 'Administrator');
$role = get_role('administrator');
$role->add_cap('manage_capabilities');
$role->add_cap('switch_themes');
$role->add_cap('edit_themes');
$role->add_cap('activate_plugins');
$role->add_cap('edit_plugins');
$role->add_cap('edit_users');
$role->add_cap('edit_files');
$role->add_cap('manage_options');
$role->add_cap('moderate_comments');
$role->add_cap('manage_categories');
$role->add_cap('manage_links');
$role->add_cap('upload_files');
$role->add_cap('import');
$role->add_cap('unfiltered_html');
$role->add_cap('edit_posts');
$role->add_cap('edit_others_posts');
$role->add_cap('edit_published_posts');
$role->add_cap('publish_posts');
$role->add_cap('edit_pages');
$role->add_cap('read');
$role->add_cap('level_10');
$role->add_cap('level_9');
$role->add_cap('level_8');
$role->add_cap('level_7');
$role->add_cap('level_6');
$role->add_cap('level_5');
$role->add_cap('level_4');
$role->add_cap('level_3');
$role->add_cap('level_2');
$role->add_cap('level_1');
$role->add_cap('level_0');