Why does traffic or conversion rate show as unavailable?

Because Shopify has not given nouz access to your store's traffic yet. Sessions come only from Shopify's own traffic reports, and without sessions there is no conversion rate to compute, so nouz shows both as unavailable, with the reason, instead of guessing.

Shopify lets an app read its traffic reports only with two things in place: a reports permission, and Shopify's approval of the app for the level of customer data access those reports require. The message on screen says which one is missing.

“nouz needs one more Shopify permission to read store traffic” means your store was connected before nouz asked for the reports permission: open nouz from your Shopify admin, under Apps, as an owner or admin, and Shopify shows its approval screen for it once. “Shopify has not granted nouz access to store traffic yet” means the permission is there, and either Shopify's approval of nouz for that data is still to come or your Shopify plan does not include the reports. “Traffic data requires Shopify Grow plan or higher” applies to a store connected through a direct install link, for which Shopify includes the access from its Grow plan upward.

Sessions are for analysis only. They feed the conversion rate on Insights and nothing on the P&L, so every other number in nouz is complete whatever the traffic says.