Note: Assigning a custom RBAC role as the Management Group level is currently in preview only. So, for now the answer to the assignable scope is the subscription level.
Reference:
https://docs.microsoft.com/en-us/azure/role-based-access-control/resource-provider-operations
https://docs.microsoft.com/en-us/azure/role-based-access-control/custom-roles
https://docs.microsoft.com/en-us/azure/role-based-access-control/custom-roles-portal#step-5-assignable-scopes