The average session length metrics in Data Studio appears to be broken. I keep getting a system error on any reports/chart formats (scorecard/timeline).
Sorry that you are now having issues with this. It was an issue we knew about from some prior reports and we deployed a fix recently that should have addressed it. However, this seems not to be the case, so I will re-escalate the issue to the development team to investigate further.
Please let us know once this issue is fixed. My clients are asking for this as most of the reports are broken at the moment.