You started the registration process and you are getting the following error message:
“No advertising seller or vendor account linked to this amazon user”
You are getting this error because you did not start Advertising with Amazon. Having started Advertising on Amazon is the main prerequisite to leveraging the technology.
Before completing your registration, you should launch your first Sponsored Product campaign on Amazon. We recommend you launch an auto campaign. Once your campaign(s) goes/go live, you can pause it right away and link your account to Astra.
More details on how to start advertising on Amazon are here.