Your best bet here is to contact support as I have seen this issue a few times.
However as a workaround, In one of the replies on a thread she was successful signing up using his mobile phone from a browser in incognito mode using mobile data, not WiFi.
You can try that as well.
To contact support call the global service numbers here: https://support.microsoft.com/en-us/topic/customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
Regards,
Abiola