Scenario 1: You attempted to pair your account using the instructions provided in this article, but your connection shows "Pending":
If your account is brand new or you recently upgraded to a Professional Seller's Account, Amazon may delay the approval of your credentials for a few hours (or even a day or two). The best advice is to try again in a few days. If you are still unable to connect your Amazon Account after a week has passed you may need to reach out to Amazon Seller Central Support.
Scenario 2: You followed the instructions provided in this article, but received an error stating, "You are not permitted to take this action. You must be the primary user of a Professional selling account to take advantage of apps. If you are the primary user you may upgrade your plan. Click here to upgrade your plan."
This error indicates that you have an Individual seller account rather than a Professional seller account. The primary difference is that Amazon Individual seller accounts do not have a monthly subscription fee. In contrast, Professional seller accounts have a monthly subscription fee of $39.99. However, individual sellers pay a $0.99 fee for each item sold. The professional plan is designed for sellers who plan to sell more than 40 items per month. You can upgrade using the link provided in the error if you choose.