Because the authorization of your seller account to tool4seller expires. The authorization of your seller account to tool4seller usually lasts one year, and periodically Amazon will send emails to ask you whether renew tool4seller’s access.
Go to your "Amazon seller central", and select the "Manage your apps". Then click on the "Renew" button in the tool4seller row.
More questions, welcome to contact us at support@messages.tool4seller.com.