Sometimes when completing the screening survey using a mobile phone, the autocomplete drop down menus take some time to appear after you enter your post code.
Give a few seconds after your enter your postcode for fields to appear and populate below the post code with your suburb and state. Then, select your state and suburb (you will continue getting the "invalid postcode" error until you select your state and suburb from dropdown menus).
Just like this:
If you do not see the dropdown fields with your suburb and state, please email us at firstname.lastname@example.org and we will help setting you up wit the account manually.