Can’t login with my email

my problem is that I downloaded the app with another username which I provided to buy the book and downloaded my university book there on my mac but I wanted to have my book on my iphone as well. I downloaded the app I can’t log in with the same username as I bought the book and downloaded on my laptop, It says There is no username or email such as that. It is so frustrating that I can not find the solution almost 2 hrs and customer service is a complicated process here. I have exam and not able to use my laptop. Someone should help