If you read the text under the results it states GFAST, FTTC and SOGFAST orders are temporarily unavilable for fibre orders due to engineering works.
Give it a few weeks and try again
It's strange as when I type in my neighbors addresses to order it, they are able to and recieve no error yet I do despite the fact we're connected to the same cabinet.
I'll try again in a few weeks though thanks for the help 🙂
the reason for difference in checker result is your neighbour has Fibre and you at present don't