You can access this as soon as your SIM is fully active. You'll need to make a £10 top-up or buy a goodybag for this to happen, and from the time you pay it takes up to 24 hours (although usually it's less than an hour).
Hi there and welcome to giffgaff.
Your sim needs to be fully activated before you can access that page to set up a number transfer.
It can take up to 24 hours for the sim activation to fully complete.
Don't worry, nothing is wrong. All that is required is a little more patience.