why would a sales order doesn't show up on the Ship Central UI?

Have you done the picking for that SO? Ship Central is planned to do the pack and (eventually, if you use Ship Engine integration) the ship steps, not the picking

Yeah

The picking has been done through normal UI or through NetSuite WMS? Sometimes the WMS has some delay before creating the item fulfillment, therefore you can’t see it into the Ship Central. First of all, have a look at the SO, does it have an item fulfillment created? Is the location the same one you’re trying to use in the Ship Central? I suppose the items included into the SO (and the item fulfillment) are Inventory/Assembly Items? If you’ve checked all the above elements, can you give us a bit more details to understand what could be the issue? For example, you can’t see any SO or only some specific SOs?

I will get back with more findings but it has to be done in UI, no NS WMS