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Two sibling Scouts in the troop; one of which has both parents listed, the other only one. When trying to add the missing parent to the singleton, the search functionality claims to be unable to find the parent despite all information being copied-and-pasted from their IA entry.
The most reliable workaround is, if the parents have a my.scouting.org account, is for them to log into Scoutbook with their ID and PW, navigate to My Account → My Connections then click the “Connect to Scout as Parent/Guardian” near the top of the page. The parent will need the Scout’s BSA Member ID, last name and date of birth to connect.
If the parent is in the unit already by a position or another Scout you can use Connection Manager under Roster, then go to the connection and click the Parent Checkbox