Getting METHOD_NOT_SUPPORTED response to billing agreement token request

[subscriber_dev]

Hi,

I'm investigating using billing-agreements REST, v1/payments/billing-agreements (which is a limited use API), and want to test in my sandbox.
However, when I run the following: /v1/payments/billing-agreements/agreement-tokens

I get the following error:

{

    "name": "METHOD_NOT_SUPPORTED",

    "message": "The server does not implement the requested HTTP method.",

    "debug_id": "f313033fea9cd",

    "details": [],

    "links": []

}

I have the options for Vault disabled and Advanced Checkout keeps resetting.

Can this be enabled, please?
My sandbox accountId is 9FHABUN2CAGNC