Why does Confluence say I don't have permission to view pages when i'm Admin?

[Rodney Hughes]

They may have set Page View Restrictions without you on the list of allowed viewers

[Nic Brough -Adaptavist-]

Probably because you don't have permission to view them.

Admin gives you admin rights (including the ability to change permissions).  It does not mean "can see everything", as that's a very poor permissions model.  If you want to see a page, you must meet the rules the space and page have set on them (those rules could, of course, include "if admin", but they don't have to)