Make sure you have completed your catalogue and your settings and either the Product Export or Product ID Sync. Only products in the catalogue that are marked as 'exportable' will have the ATS figure exported to Shopify.
Click on the 'Stock -Export' green arrow
Available To Sell stock levels will be sent to Shopify from the season(s) you have selected in your Connector Settings.
You can leave this page. This process will be running in the background.