We ran into an issue today where a Farm Administrator could not add an app to a SharePoint site.  When visiting the page to add the app it said:

Sorry, only tenant administrators can add or give access to this app. 

A Farm Administrator is, by definition, a tenant administrator so this was a bit odd.  Jeff Burt found a random post in a forum (sorry I do not have the link) where someone said that adding the user to the local administrators group on the SharePoint server itself made this issue go away – and it totally does.  Not an ideal solution, but it is a solution nonetheless.