Error updating scheduled query: update_time cannot be updated

This is an issue already reported to the BigQuery engineering team.

You can track the progress of the research regarding this issue in this link. It seems an issue with the BigQuery UI as the Classic UI seems to be working

I suggest starring the issue to indicate you're being affected by this issue.

UPDATE

The classic UI may have issues regarding the dataset destination set.

I consider recreating the scheduled query would be the best workaround while the fix is released.