I think the error is being triggered by your trying to set both the Status and the Contract Effective Date. A subscription in Draft has no trigger dates set, so if you drop the Contract Effective Date, the create call should work and your subscription will be in Draft. As Adam noted however if you use the REST you may HAVE to specify the Contract Effective Date, meaning you can't actually set a subscriptin in Draft, which I'd say is a bug as it's inconsistent with past behavior.
This may not have been clear from Adam's answer, but the Status field on the Subscription object is normally set by Zuora automatically, not explicitly set as you are trying to do. The Status based state model for the Subscription object is, Draft, Pending Activation, Pending Acceptance, Expired, Active, Suspended and Cancelled. No trigger date on creation will put the subscription into Draft (you can see this in the UI, in your sandbox create a subscription, add rate plans, save without specifying Contract Effective or any other trigger date, you'll have a Subscription in Draft). If you configure your tenant to require Service Activation Date be populated (this is optional and may not be set on your tenant), then you can set the Contract Effective Date, NOT set the Service Activiation Date and you'll end up with a Subscription in 'Pending Activation' status. If you just require Contract Effective (the OOTB default) then once you do set the Contract Effective Date on that Draft subscription the status will change to 'Active'. When you create an amendment for the subscription, that first version that was active goes to 'Expired', the second version of the subscription (which is a new Subscription object) acquires the Active status. You can probably guess what Suspended and Cancellation means!
