Authorizing a device for iBooks usage

Hello!

I'd like to authorize my MBP to be used with iTunes. When I click on 'Store' and then on 'Authorize this Computer', I get an error saying: "You can only authorize 5 computers. You have already authorized 5 computers with this Apple ID. To authorize this computer, you must first reauthorize one of the other computers."

So I did exactly that.

I went to appleid.apple.com and I removed the devices that I no longer own. I currently have the following devices on the account: my iMac, MBP, iPhone, Watch, Apple TV, iPad, and a Windows desktop. So my MBP is already on the account but I am still getting that error. I even tried restarting the MBP to no avail.

What should I do?
Richard