This error can be confusing as users think that it means the Scout is not on the official BSA roster. This error actually means that the Scout is not set up for the advancement sync in Scoutbook. This can be corrected by having the user click on the Scout > Scout’s Membership > Click on the current Membership > Uncheck Position Approved > Update. Then have them go back in and check the Position Approved checkbox again > Update. That should set them up for the sync and the error message will go away.
More on Scout for Advancement Sync and Scout not on official roster problems can be found here.