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, let them run for at least 10 to 15 days before registering.
More details on how to start advertising on Amazon are here.