Open the report's Preview from Reports. If it shows an error, wait a few minutes and open it again: a temporary problem often clears on its own. If it's a manual report, don't use Send report to retry: it emails the report to every recipient.
Check the dashboards the report is built from. If one of its widgets shows an error there, the report can't include it either.
If it still fails, contact us with:
the report's title,
the property or properties it covers,
the date you tried,
and a screenshot of the error.
[Image: A report's preview page showing the error]