Hi @mpmc sorry you are having trouble setting up the Cloud. I moved your post to create your own thread as that's the best thing to do.
Contact the Cloud helpdesk on 0800 500 3114 and they will be able to help you.
They are open 9am to 9pm Mon - Fri and 9am to 6.30pm Sat and Sun.
Post back and let us know how you get on.
I have this issue too despite being encouraged by BT to receive boost and activate BTCloud.
This I have tried to do on an iPad running the most uptodate OS for the model, on an iPhoneSE running the most recent OS and on a macMini running the most recent OS. I get this same message.
I have different cookie settings/rowser on the iPad and iPhone cf the MacMini, so the issue would seem to be compatibility with Macs I hazard.
The MacMini is shared with my child so I wanted to use BTCloud for items I did not want on the same computer (my MacBook is being repaired) to prevent deletion of these items etc. or documents re personal info. I may try to install it on the one shared Windows 10 laptop we have but i find the Windows interface problematic on several grounds.
I appprecite that we will not be able to resolve this before going to Germany late tomorrow, but even so my phoning a helpline is not an option as I have Autism and struggle with using phones. I have tried the chat window on BT help but they seem unable to help.
This error seems to have recurred (as evidenced by my searching here and elsewhere) over at least the past year, so a fix should have been found.