As an admin, when I try to edit an onboardee I see only the "view" button. Why is that?

That is probably happening because you are not a super admin. There are 2 main causes of that:

1. Your admin rights don't allow you to edit onboardees;

2. Your admin rights allow you to edit onboardees but your filters don't match to the ones of the onboardee.

In both cases the best to do is to contact the Super Admin of the app and her/him to have it changed if possible.