Pardot API must be updated to use a Salesforce user instead of a Pardot user.

ACTION REQUIRED: On February 15, 2021, Pardot's user authentication system will be discontinued. Going forward, all users must use Salesforce single sign-on (SSO). Pardot users who are not enabled with Salesforce SSO by February 15, 2021 will not be able to log into Pardot until they are connected to a Salesforce user. Check out the Pardot User Migration Admin Guide, Pardot User Migration FAQ, and series of topical videos for more information.