iBooks 1.0.1 on OS X 10.9.5 Mavericks won't let me sign in to my Apple ID/Account

I am trying to sign in to iBooks to download a few books to read. I must sign in to be able to buy/get books, which is fine. However, after I type in my email address and my password, I get the following error box:

Apple Account creation is not currently available on this app.

To create an Apple Account, go to iTunes on your computer or iOS device.

I can sign in perfectly fine without a 2FA code on both iTunes 12.6.2.20, and App Store 1.3 (OS X Mavericks). I am not trying to create an account, but instead, I am trying to log in to my current account which has already been created. If I select the option to create an Apple ID, it shows a form for me to fill out in order to create my account. I am able to make an account, but I am unable to sign into my current account. I remember being able to sign into my account about roughly a month ago, before I reinstalled OS X.

Any fixes?