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.