I don't have Book store

My Books app looks only like a PDF viewer. I cannot download any book simply because I don't have a book store (picture below). I only have the Search option and Library option which only contain PDFs that I have downloaded on my phone. The situation is the same on my iPhone and iPad (everything is synched on iCloud).

I have tried Settings > Screen Time > Content & Privacy Restrictions > Allowed Apps > Books - did not help.

Is this something related to regional settings? My region is Serbia.