I am looking to overwrite this message via functions.php through a child's theme. I have seen some examples through here and here.
I would prefer using a filter but don't know why is it not working when I add it into functions.php:
add_filter( 'woocommerce_registration_error_email_exists', function( $html ) {
$url = wc_get_page_permalink( 'myaccount' );
$url = add_query_arg( 'redirect_checkout', 1, $url );
$html = str_replace( 'Please log in', '<a href="'.$url.'"><strong>Please log in</strong></a>', $html );
return $html;
} );