Jack,
the reason you're not finding the metric is probably because you haven't created the log-based metric. Note the "create metric" button right under the top red box in your screenshot - did you try that?
That being said, with app engine there's an easier way: app engine already counts your 5xx errors and ingests this as ts (in addition to many other metrics). What you could do is simply create a policy, choose app engine as the resource target, and from the metric dropdown choose 5XX responses. LMK if that works for you.