Why does Analytics show my sales but say "Collecting sessions" for traffic?

Nothing is broken. Analytics → Dashboard is built from two sources:

• Sales widgets (Total sales, Orders, Avg order value, Total sales by product and the rest) come from your orders, so they show data straight away.
• Traffic widgets (Sessions, Conversion rate, Sessions by device type, location, landing page and referrer) come from WCart's built-in storefront tracking. It records page views as shoppers browse. You don't need to install anything.

Until visits are recorded for the date range you picked, those boxes show Collecting sessions… (or device data and so on) with "This fills in once your storefront starts sending analytics". Once shoppers start visiting, the boxes fill in.

Try widening the date range at the top. Use All reports → for the full report list.