Oracle Error Message
ORA-13707: Either the start snapshot string or the end snapshot string is incomplete or missing key statistics.
Reason for the Error
Either the start or the end snaphots was missing or purged or had encountered errors while creating them.
Verify that AWR is done taking these two snapshots, or Look in DBA_HIST_SNAP_ERROR to find what tables are missing in the start/end snapshots. Use the ERROR_NUMBER column in that view together with the alert log to identify the reason for failure and take necessary action to prevent such failures in the future. Try running ADDM on a different snapshot range that has valid start and end snapshots.