I’m a mobile user, I had previously purchased several books on the mobile app but switched accounts because the one I did the purchases on I no longer have reliable access to. 
switched to a gmail account and tried to purchase again but I get a prompt saying I have bought this book before and if I want to get it for free. I press the ok button and a long loading screen appears but nothing changes afterwards.

Try contacting support I think they’re the only folks who can help you. Good luck though, I have been waiting on customer support for a month.