When a summative report is completed, the Final Score is recorded in the database. If additional data is added later on (e.g. a form is marked 'complete' that was 'in progress' at the time the summative was completed), or a rating on a goal has been changed, then the summative calculation also changes.
Here's what you'll need to do:
To update the summative report, click Actions > Re-open for Editing. The Final Score should update, and the error message dismissed. You can then complete your report again and re-share as needed!