iTunes won't acknowledge my Username / Password on my Macbook.

I'm signed in to iTunes on my iMac. When I attempt to sign in to iTunes using exactly the same Username and Password as my iMac nothing happens - there's not even an error message, I simply remain signed out despite the Username and Password having seemingly been accepted.

Can anyone assist please? Cheers.

MacBook Air (13-inch, Early 2015)

iMac (27-inch, Late 2013)