Apparently I've gone and broken the build. As a newbie, I have no idea how fix this. The connection between my change and this problem is unclear to me. Any help would be appreciated.
The problem went away (bot is green again), so it might not have been you:
Do I need to click the Rebuild button on the results page?
No. It shows green for a later build already. Unless if you really think that your change introduces a transient issue, there’s nothing to follow up on with regards to your change in response to the failure report.