First, check if your Shopify account has sent a fulfillment request to Supliful.
By default, Shopify stores are set to Manual Fulfilment; you would need to either send each order to Supliful manually by clicking the Request Fulfillment button:

You can set your Shopify store to Automatic Fulfillment by visiting the Shopify Settings --> Checkout. There, scroll down to the Order Processing section, and under the heading, After an order has been paid, select Automatically fulfill the order's line items. Don't forget to click the Save button when you are done:

Sometimes, during downtime, some order requests are not processed by Shopify, and we don't receive your order requests. Even though we regularly run scripts to ensure we pull all the details in, we cannot do that on some occasions, as Shopify did not forward those to us in the first place.

When sending a fulfillment request to Supliful automatically or manually, you should see a message in the order log saying Supliful has accepted your fulfillment request:

In case you do not see that message, you will need to cancel the fulfillment request first, then send the Fulfillment request again: