If you're locked out of your NHSmail account, you can go to this NHS help site to find out how to reset it or unlock your account.
We send accuRx Chain verification emails to NHSmail accounts, so you'll need to be logged in in order to use Chain. We also send magic link/reset password emails, and optional blog posts and weekly updates to your NHSmail account, so you'll miss also these if you can't log in.
Don't have an NHSmail account? See here.