It's a very strange issue. Look to the error message, it says "Call to undefined function error_log()" that means "error_log" function was not found. However it's a native PHP function which should be present always.
Why it's not defined? It can be your server or your hosting issue. Could you consult your hosting provider's support for that?
Also there is a link to this code from WPFTS core:
$q = 'select
sum(if ((force_rebuild = 0) and (build_time != 0), 1, 0)) n_actual
where `tsrc` = "wp_posts"';
$res = $wpfts_core->db->get_results($q, ARRAY_A);
As you can see it's a very simple code that basically should not trigger a critical error.
Please could you tell me which WPFTS version you tried and which PHP version is used there.