Plugin activation fails

When I try to activate the plugin, I see the following error message:

The plugin generated 447 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

Subsequently, when using any of the Term Order screens I see this:

WordPress database error: [Unknown column 't.term_order' in 'order clause']
SELECT t.*, tt.* FROM wp_terms AS t INNER JOIN wp_term_taxonomy AS tt ON t.term_id = tt.term_id WHERE tt.taxonomy IN ('category') AND tt.parent = '0' ORDER BY t.term_order ASC

The column ‘term_order’ does not exist in the database. I think it is because the SQL to create the column in the table includes tags (customtaxorder.php, line 424):

ALTER TABLE $wpdb->terms ADD <code>term_order</code> INT( 4 ) NULL DEFAULT '0'

The plugin functions properly when I change that line to:

if ($init_query == 0) { $wpdb->query("ALTER TABLE $wpdb->terms ADD term_order INT( 4 ) NULL DEFAULT '0'"); }

https://www.ads-software.com/plugins/custom-taxonomy-order-ne/