You are in the US, signing up with a US address? If so, and you are still having issues, I advise you to contact Project Fi support. If you go to the Project Fi Help start page, you can click on the “Contact us” link at the top right of the screen, then select “Basics and Signup.” From there, select whether you want to receive a call back, an email, or open a chat. Or you can call 1-844-TALK2FI to connect directly.
If you are not within the US, and/or are trying to sign up for Fi using a non-US address, you will not be able to sign up for Project Fi.