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 who had multiple accounts. They are now merged and it was supposed to be connected to his scouts account. But it’s been almost a month since the merge and his account hasn’t been connected to his sons.
In principle, a unit Scoutbook admin can connect parents to their respective scouts. Generally the unit Key 3 (CM/SM, CC, COR) are all Scoutbook unit admins, and they (or other unit admins) can assign additional admins.
The information at the help link below is slightly dated, but still appears accurate. A key issue is making sure that you search for the adult (I recommend using email address since it’s unique), since you know they already have an account. If you don’t find the parent via that method, it might be worth checking to see if they are already connected to the scouts, just not as a parent.