Parent Search is broken - watch Change Log
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 does not have a my.scouting.org ID, they should create one then use the process above.
Alternately, you can ask your Council to add a connection between the Scout and parent in Registrar Tools. About 24 hours later, the Scout & Parent will be connected. The parent will still need a my.scouting.org ID to access Scoutbook.