Trying to run a advertising campaign?

Hi all i am a new seller on amazon, haven’t had a sale as of yet.
just wanted some help on running the advertisement, it says not eligible because you don’t have the buy box? any explanation or help on this would be great, i am the only person selling this brand of shampoo.

thank you in advance

Sellers have to qualify for the buy box - its the box top right of an amazon product page you can often see for products where as a buyer you can order with one click.
To qualify there are various elements involved - amazon will not discuss all that’s included.
And for new sellers you have to have been selling for 90 days plus.