You cannot use the “Activate Your Account” button on the main Your Health File login page to create a new account, you must use the link in the email from Your Health File
Search your email box (including spam/junk) for the email from Your Health File - sent from HFAlerts@nextgen.com
It should look like the picture below
Click the button that says "click here to begin the registration process" and you will be asked for your date of birth:
Once you input your date of birth, it will allow you to set a username and password
After you set your password, you will need to set up 2-Factor Authentication using text or email