Your ad is not eligible for impressions because your listing is not in the buy box. This message keeps coming up when I enter my ad campaign???

If you have an item where top right of product page has a box for buyer to click to order, thats the buy box.

To qualify you need decent metrics, time selling 90 days plus or FBA.
And metrics in good standing - a single negative feedback can unqualify your account for weeks or months. Or have no impact if enough sales.