"activate config" with SEQNO or SCN option does not work if that SEQNO happens to be from an archive log which is prior to the archive log generated after RESETLOGS during recovery of the Source database or is an SCN from such archive log.
It fails as below (shown here when activating with the SCN option):
sp_ctrl> activate config config_file_name SCN=10031522975552
Error: Unable to activate config at 10031522975552
The following is an example of what might be encounterd in the Source event_log file:
Info 2019-06-20 14:50:56.894498 29983 1 Activation exited with code=1, pid = 204 (activating datasource o.SOURCESID)
Error 2019-06-20 14:50:56.873254 204 1 Compile of "config_file_name" for database o.SOURCESID failed: Error: Unable to activate config at 10031522975552
Error 2019-06-20 14:50:56.871280 204 1 Activation: Failed to find redo sequence# associated with recovery SCN 10031522975552 for instance: 'SOURCESID' thread: '1'. ORA-01405: fetched column value is NULL (activating datasource o.SOURCESID) [module ocf]
Notice 2019-06-20 14:50:54.987583 204 1 Activation: Activation for recovery to SCN 10031522975552 started (activating datasource o.SOURCESID) [module ocf]
Info 2019-06-20 14:50:54.874071 204 1 Activation launched, pid = 204 (activating datasource o.SOURCESID)
Notice 2019-06-20 14:50:54.869732 29983 1 Cop: Initiating recovery activation (scn=10031522975552) [module cop]
Similar output would be generated if the SEQNO option were to be used instead of the SCN option.