WP-Cron function not firing

Looking at your function:

function my_task_function() { 
    $test_score
    update_field('score', $test_score );
}

$test_score is not defined here, it should be $test_score=”something”; Left the way you have it now will generate php warnings.

Also the update_field function used in this context (in a cron) will require you to pass a third argument specifying which post/page contains the field that you want to update. See the full docs on that function here: Advanced Custom Fields: Update Field.