The most likely cause of this is that the activation link has expired (i.e. more than an hour has passed since the email was sent).
To get your account activated, you can:
- Navigate to Log in to NHS Digital developer account and sign in with the account created (but not activated) previously.
- You see the screen about verifying your email address again.
- A new email is sent automatically.
- Click on the activation link in the new email within an hour.