QuickBooks Online (QBO) uses the pre-built address data specific to the United Kingdom. To validate your address with the correct postcode, I suggest using this postcode finder: https://www.postoffice.co.uk/postcode-finder.
Once verified, try setting up your company again. If the same thing happens, try accessing QBO on a private or incognito window. It'll help us check if this is a browser issue or not.