question

The order ID which you pass to Register must be a Clover order ID for an order that exists in Clover's system.

It's not clear what you are attempting to accomplish so it's hard to give you a useful answer here. It sounds like your system has already created it's notion of the order. I'd assume either 1) you want to use Clover to pay for the order only and don't care if the order actually exists in Clover or 2) you want to create an order in Clover to match your notion of the order.

If it's (1), you want to start Payments, not Register. Register is for building orders. See ACTION_CLOVER_PAY in the Clover Android SDK. Note that you'll still need to at least create an Clover order and give it one line item that matches the amount of your order. Alternatively, if you just want to accept card payments, you can call Secure Payments directly. See ACTION_SECURE_PAY for that one. In that case, per the docs, you can omit an order ID. An empty order will still be created in the Clover backend but you don't need to worry about ot.

If it's (2), you need to use the Clover SDK class OrderConnector to create an order and populate it to match your notion of the order. You can then use either ACTION_CLOVER_PAY or ACTION_SECURE_PAY (see above).