WARN[0040] is not an error code, it’s a warning of a failed HTTP request that happens at the 40th second of the script execution 
Not the best UX, I’d admit, but if you run k6 with --verbose, you’d see a lot more log messages and see this isn’t something special.
Regarding the actual error, it might be caused by any number of things. Maybe k6 overloads your system under test, maybe a network issue, maybe an HTTP2 issue with the server 
I’d suggest upgrading the k6 version to the latest v0.27.0 and not worrying about a single HTTP request failure, unless it’s consistent. And if it is, investigate the issue with the specific URL on the server side?