WPML icl_register_string() throws fatal error but works [closed]

You should always use the wpml functions as following: (the HowTo linked by you actually suggests this 🙂

if( function_exists('icl_register_string') ) { icl_register_string( 'Match Previous Order' , 'match_order', 'Do you want these items to match a previous order from Direct Linen? If yes, use "Additional Notes" to explain.' ); }

This will ensure that you won’t get any errors in case WPML doesn’t exist in a WordPress Installation or WPML wasn’t fully loaded before your code is run (this is the case when your plugin is activated).