For the United States, please click the Marketing section on the left and then click the Ads section on the left.
Please ensure that you have logged into Seller Center using a user email address that has Shop Owner or Admin or Advertising Manager permission. If you do not have this Seller Center permission, please request your Shop Owner or Admin to grant you this Seller Center permission.
After being granted this Seller Center permission, if you still cannot see the Ads tab on the left, please clear your browser cache & cookies, restart your browser, log into Seller Center again, and go to the Ads tab again.
This means that your shop has not completed the process to start selling products. Your shop should first complete the necessary process in the Seller Center and have a product available for purchase.
After that, you should no longer see this error message. Please complete the necessary steps given on the page and then click the Create Campaign button.