The Smart Send Premium app for Shopify is only designed for stores using one Location ID. If you have more than one Location ID in your Shopify admin (Shopify>Locations menu) then you cannot use this app.
Alternatively if you only need one Location ID (because you have only one pickup address for your shipments), you can remove any extra Location ID addresses from the Locations admin menu in Shopify and you will be able to fulfill any new orders (from that point on) through the app.
Also check that you have funds in your Smart Top-up account in your Smart Send VIP account console on the Smart Send website (smartsend.com.au). You can add funds to your Smart Top-up account via credit card, Paypal or Bank Transfer. Once funds are available you will be able to fulfill orders through the Smart Send Premium app seamlessly.