Appstore Endless Loop - iPhone 6s

Hi,

Please advise me if you can. I'm still quite new to smartphones.

I cannot get any Apps at all.

I open the Appstore and search for an App, say WhatsApp.

I click on ‘Get’ and a screen appears which says ‘Sign in to Complete Purchase’.

I insert the correct email for my Apple ID and my correct password.

Another screen appears.

It says ‘The Apple ID has not yet been used in the iTunes Store’ and ‘Tap Review to sign in.’

I tap Review. It then says ‘Use Existing Apple ID’. I tap this.

Another screen appears. It says ‘Sign in Required - Please enter your Apple ID and password to create an iTunes Store account from your existing Apple ID’.

I retype my Apple ID and password as requested.

It returns to ‘Review’ screen.

This continues in an endless loop.

I give up.

Does anyone know what I am doing wrong please?

Thanks