Authorization

I’m having trouble with authorizing my new Mac to view previous iBook purchases (school textbook). I’ve fallowed the steps given online but when I go to my iTunes settings there isn’t an option to “authorize this mac”

it tells me there is 2 computers with authorizations and gives me an option to deauthorize them but nothing more

i can view this book on my phone but not on mac

help, I need this textbook asap