An order that has a status of 'Assembling' in Omnivore means that the order has been created in Catch and Omnivore can see it but its not completely ready to be processed and sent to your ecommerce store for fulfilment.
Omnivore will accept the order while Catch verify for payment and fraud checks. Often, when an order is in a status of 'Assembling' not all order information/data such as name and address is available.
When the order has passed all checks or any other restrictions and is ready to be fulfilled, the order status will change in Omnivore to 'Created' and then sent to the retailers ecommerce system for fulfilment.
It can take a few days for an assembling order to be sent through to the ecommerce platform.
Here is an example of what it looks like in Omnivore: