Can't sign into app store - asking for verification code
I suddenly can't sign into the app store on my Mac 10.9.5.
When I enter my Apple ID and password I get a red message saying:
'An Apple ID verification code is required to sign in. Type your password followed by the verification code shown on your other devices.'
and that's it.. there's no option to generate a verification code and I haven't received one through this dialogue.
In an attempt to remedy this I went into my iCloud account in system preferences and somehow generated a verification code.
I tried typing my password again, followed by the verification code, to no avail.
I'm going round in circles here - hope someone can help..
Mac Pro, 10.11