When entering an address in the postcode lookup box and after clicking “find my address”, you should see a list of addresses which, after clicking the appropriate one, will complete the fields in the address section. See 10 second video below.
If you’re not seeing the behaviour as shown, the most likely reason is that your browser is not the latest version and needs updating. It could also be if you have ad-blockers in your settings which is blocking the list of addresses.
It could of course be that the postcode doesn’t exit! You can check the postcode is correct via Royal Mail here.
Please see below for help.