First time iphone user forced into itunes, but next button greyed out

I'm a first time iPhone user, and whilst I only have an iPhone8 plus, it's enough for my needs. I've gone to the app store for the first time, and after trying to install something, I'm prompted for my password, no problem, but then before it installs it says the apple account has not yet been used in the iTunes Store, which seems to be a prerequisite? I click review, and then select to agree to the terms and conditions, but afterwards the blue 'next' button is greyed out, and I can't progress any further. I've tried switching the phone off for a while, logging out and back in, but I can't get past that screen and therefore can't download anything. Very frustrating experience.