Cant sign in to App Store on Mojave and install/update apps

Hello Everyone,

I recently purchased iMac and everything seems to running fine expect App Store. I can't sign in to App Store and install or update apps. When I click on Sign In, it asks for my iCloud ID and password and literally nothing happens when I enter it. I am sure I have been entering it correctly and my Internet is also working.

Interestingly, the problem is with the App Store only and not with iTunes/Books app.