Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
I have a parent with two IDs 131821936 and 141757203. The parent can login via Apple ID to 131821936 but not to 141757203. The scout ID 141757209 is associated in Scoutbook with the parent account that cannot be accessed.
@JeffreyAltman these MIDs are setup to be managed in My,scouting.org > Manage Member ID - the parent needs to go there and change the one marked PRIMARY to log into the other account. Or they could merge them, but we do not really advise that at this time.