Trying to sign into Mail app on Mac, wont let me Add Account.

So, I'm trying to sign into my iCloud account in the mail app, but once I get to the part where I select the apps I want to use my iCloud account with, it doesn't do anything when I press 'Add Account'. I can press it 50 times and it won't do anything. It shows the animation when I click it, but doesn't bring me anywhere, nor sign me in. I am on the latest Software Update, and have tried restarting many times. Nothing works.